docs: Add browser support guidelines (#23457)

## **Description**

A document about browser support guidelines has been added. This
document describes our browser support policy, and the notification
requirement when updating the minimum supported browser versions. This
is not a new policy, it just wasn't written down before now.

## **Related issues**

Relates to #6805

## **Manual testing steps**

N/A

## **Screenshots/Recordings**

N/A

## **Pre-merge author checklist**

- [x] I’ve followed [MetaMask Coding
Standards](https://github.com/MetaMask/metamask-extension/blob/develop/.github/guidelines/CODING_GUIDELINES.md).
- [x] I've clearly explained what problem this PR is solving and how it
is solved.
- [x] I've linked related issues
- [x] I've included manual testing steps
- [x] I've included screenshots/recordings if applicable
- [x] I’ve included tests if applicable
- [x] I’ve documented my code using [JSDoc](https://jsdoc.app/) format
if applicable
- [x] I’ve applied the right labels on the PR (see [labeling
guidelines](https://github.com/MetaMask/metamask-extension/blob/develop/.github/guidelines/LABELING_GUIDELINES.md)).
Not required for external contributors.
- [x] I’ve properly set the pull request status:
  - [x] In case it's not yet "ready for review", I've set it to "draft".
- [x] In case it's "ready for review", I've changed it from "draft" to
"non-draft".

## **Pre-merge reviewer checklist**

- [ ] I've manually tested the PR (e.g. pull and build branch, run the
app, test code being changed).
- [ ] I confirm that this PR addresses all acceptance criteria described
in the ticket it closes and includes the necessary testing evidence such
as recordings and or screenshots.

README.md

@@ -229,6 +229,7 @@ Whenever you change dependencies (adding, removing, or updating, either in `pack
 - [Developing on MetaMask](./development/README.md)
 - [How to generate a visualization of this repository's development](./development/gource-viz.sh)
 - [How to add new confirmations](./docs/confirmations.md)
+- [Browser support guidelines](./docs/browser-support.md)
 
 ## Dapp Developer Resources
 

docs/browser-support.md

@@ -0,0 +1,14 @@
+# Browser Support
+
+MetaMask's browser support policy is to support:
+
+* The last two years of Chrome stable releases
+* The current and previous Firefox Extended Support Release
+
+We try to support browser forks as well (aligned with supported Chrome/Firefox releases), but we don't actively test them.
+
+## Providing notice of minimum supported version changes
+
+Additionally, before bumping our minimum supported browser versions, we should always notify users in the application first. Otherwise users of older browser versions would unknowingly get stuck on old MetaMask versions, even if they had automatic updates enabled.
+
+We have a home screen notification already for this purpose. All we need to do us update the `OUTDATED_BROWSER_VERSIONS` constant in the release prior to the minimum version bump, and all affected users should be notified in-app.