Introduce next Changelog file (elastic#9767)

This introduces CHANGELOG.next.asciidoc and
CHANGELOG-developer.next.asciidoc. These changelog files will document
yet unreleased changes.

Having a separate changelog file reduces the work required to cleanup
the changelog, as rebasing and cherry-picking on backports sometimes
moves changelog entries into the wrong place.

CHANGELOG-developer.asciidoc

@@ -13,9 +13,9 @@ other Beats should be migrated.
 Note: This changelog was only started after the 6.3 release.
 
 === Beats version HEAD
-https://github.com/elastic/beats/compare/v6.3.0..master[Check the HEAD diff]
+https://github.com/elastic/beats/compare/v6.3.0..v7.0.0-alpha2[Check the HEAD diff]
 
-The list below covers the major changes between 6.3.0 and master only.
+The list below covers the major changes between 6.3.0 and 7.0.0-alpha2 only.
 
 ==== Breaking changes
 

CHANGELOG-developer.next.asciidoc

@@ -0,0 +1,25 @@
+// Use these for links to issue and pulls. Note issues and pulls redirect one to
+// each other on Github, so don't worry too much on using the right prefix.
+:issue: https://github.com/elastic/beats/issues/
+:pull: https://github.com/elastic/beats/pull/
+
+This changelog is intended for community Beat developers. It covers the major
+breaking changes to the internal APIs in the official Beats and changes related
+to developing a Beat like code generators or `fields.yml`. Only the major
+changes will be covered in this changelog that are expected to affect community
+developers. Each breaking change added here should have an explanation on how
+other Beats should be migrated.
+
+Note: This changelog documents the current changes which are not yet present in
+an actual release.
+
+=== Beats version HEAD
+https://github.com/elastic/beats/compare/v7.0.0-alpha2..master[Check the HEAD diff]
+
+The list below covers the major changes between 7.0.0-alpha2 and master only.
+
+==== Breaking changes
+
+==== Bugfixes
+
+==== Added

CHANGELOG.next.asciidoc

@@ -0,0 +1,120 @@
+// Use these for links to issue and pulls. Note issues and pulls redirect one to
+// each other on Github, so don't worry too much on using the right prefix.
+:issue: https://github.com/elastic/beats/issues/
+:pull: https://github.com/elastic/beats/pull/
+
+=== Beats version HEAD
+https://github.com/elastic/beats/compare/v7.0.0-alpha2...master[Check the HEAD diff]
+
+==== Breaking changes
+
+*Affecting all Beats*
+
+- Update add_cloud_metadata fields to adjust to ECS. {pull}9265[9265]
+- Automaticall cap signed integers to 63bits. {pull}8991[8991]
+- Rename beat.timezone to event.timezone. {pull}9458[9458]
+- Use _doc as document type. {pull}9056[9056]{pull}9573[9573]
+- Update to Golang 1.11.3. {pull}9560[9560]
+
+*Auditbeat*
+
+*Filebeat*
+
+- Modify apache/error dataset to follow ECS. {pull}8963[8963]
+- Rename many `traefik.access.*` fields to map to ECS. {pull}9005[9005]
+
+*Heartbeat*
+
+- Remove monitor generator script that was rarely used. {pull}9648[9648]
+
+*Journalbeat*
+
+*Metricbeat*
+
+*Packetbeat*
+
+- Adjust Packetbeat `http` fields to ECS Beta 2 {pull}9645[9645]
+  - `http.request.body` moves to `http.request.body.content`
+  - `http.response.body` moves to `http.response.body.content`
+
+*Winlogbeat*
+
+*Functionbeat*
+
+==== Bugfixes
+
+*Affecting all Beats*
+
+- Enforce validation for the Central Management access token. {issue}9621[9621]
+
+*Auditbeat*
+
+*Filebeat*
+
+*Heartbeat*
+
+- Made monitors.d configuration part of the default config. {pull}9004[9004]
+
+*Journalbeat*
+
+*Metricbeat*
+
+*Packetbeat*
+
+*Winlogbeat*
+
+*Functionbeat*
+
+==== Added
+
+*Affecting all Beats*
+
+- Update field definitions for `http` to ECS Beta 2 {pull}9645[9645]
+
+*Auditbeat*
+
+- Add system module. {pull}9546[9546]
+
+*Filebeat*
+
+- Added module for parsing Google Santa logs. {pull}9540[9540]
+- Added netflow input type that supports NetFlow v1, v5, v6, v7, v8, v9 and IPFIX. {issue}9399[9399]
+- Add option to modules.yml file to indicate that a module has been moved {pull}9432[9432].
+
+*Heartbeat*
+
+- Fixed rare issue where TLS connections to endpoints with x509 certificates missing either notBefore or notAfter would cause the check to fail with a stacktrace.  {pull}9566[9566]
+
+
+*Journalbeat*
+
+*Metricbeat*
+
+- Add `key` metricset to the Redis module. {issue}9582[9582] {pull}9657[9657] {pull}9746[9746]
+- Add `socket_summary` metricset to system defaults, removing experimental tag and supporting Windows {pull}9709[9709]
+
+*Packetbeat*
+
+*Functionbeat*
+
+==== Deprecated
+
+*Affecting all Beats*
+
+*Filebeat*
+
+*Heartbeat*
+
+*Journalbeat*
+
+*Metricbeat*
+
+*Packetbeat*
+
+*Winlogbeat*
+
+*Functionbeat*
+
+==== Known Issue
+
+

docs/devguide/contributing.asciidoc

@@ -27,7 +27,7 @@ You only need to sign the CLA once.
 https://help.github.com/articles/using-pull-requests[submit a pull request] using our
 <pr-review,pull request guidelines>. In the pull request, describe what your changes do and mention
 any bugs/issues related to the pull request. Please also add a changelog entry to
-https://github.com/elastic/beats/blob/master/CHANGELOG.asciidoc[CHANGELOG.asciidoc].
+https://github.com/elastic/beats/blob/master/CHANGELOG.next.asciidoc[CHANGELOG.next.asciidoc].
 
 [float]
 [[adding-new-beat]]
@@ -147,4 +147,4 @@ In most cases `govendor fetch your/dependency@version +out` will get the job don
 
 To keep up to date with changes to the official Beats for community developers,
 follow the developer changelog
-https://github.com/elastic/beats/blob/master/CHANGELOG-developer.asciidoc[here].
+https://github.com/elastic/beats/blob/master/CHANGELOG-developer.next.asciidoc[here].