chore: add CONTRIBUTING.md

Add instructions relating to PR flow and commit
messages. These have been unformalised so far. It
is a bit nasty to ask for changes in PRs when these
have been nowhere to find. Partially copied from
Angular DEVELOPERS.md.

CONTRIBUTING.md

@@ -0,0 +1,59 @@
+###  Submitting a Pull Request (PR)
+Before you submit your Pull Request (PR) consider the following guidelines:
+
+1. [Fork](https://help.github.com/articles/fork-a-repo/) the repo.
+1. Make your changes in a new git branch:
+
+     ```shell
+     git checkout -b my-fix-branch master
+     ```
+
+1. Create your patch.
+1. Commit your changes using a descriptive commit message that follows our
+  [Angular commit message conventions](https://github.com/angular/angular.js/blob/master/DEVELOPERS.md#commits). We are not 100% strict about this, but it really helps when reviewing the PR and in making commit history readable. Use the imperative, present tense: "change" not "changed" nor "changes". The body should include the motivation for the change and contrast this with previous behavior.
+
+1. Push your branch to GitHub:
+
+    ```shell
+    git push origin my-fix-branch
+    ```
+
+1. In GitHub, send a pull request to `signalk-server-node:master`.
+* Use the same guidelines as commit messages to write the PR title and description. The server changelog is automatically generated from PR titles, so think about how you can make the changelog informative and easy to understand. The description should tell how the change affects the server's behavior and motivation for doing the change.
+* If we suggest changes then:
+  * Make the required updates.
+  * Rebase your branch and force push to your GitHub repository (this will update your Pull Request):
+
+    ```shell
+    git rebase master -i
+    git push -f
+    ```
+
+
+#### After your pull request is merged
+
+After your pull request is merged, you can safely delete your branch and pull the changes from the main (upstream) repository:
+
+* Delete the remote branch on GitHub either through the GitHub web UI or your local shell as follows:
+
+    ```shell
+    git push origin --delete my-fix-branch
+    ```
+
+* Check out the master branch:
+
+    ```shell
+    git checkout master -f
+    ```
+
+* Delete the local branch:
+
+    ```shell
+    git branch -D my-fix-branch
+    ```
+
+* Update your master with the latest upstream version:
+
+    ```shell
+    git pull --ff upstream master
+    ```