Document release branches (#1995)

* Document release branches

* Update doc/CONTRIBUTING.md

Co-authored-by: Asher <ash@coder.com>

Co-authored-by: Asher <ash@coder.com>

Author: nhooyr

.github/pull_request_template.md

@@ -1,4 +1,6 @@
 <!--
 Please link to the issue this PR solves.
 If there is no existing issue, please first create one unless the fix is minor.
+
+Please make sure the base of your PR is the master branch!
 -->

doc/CONTRIBUTING.md

@@ -2,6 +2,7 @@
 <!-- DON'T EDIT THIS SECTION, INSTEAD RE-RUN doctoc TO UPDATE -->
 # Contributing
 
+- [Pull Requests](#pull-requests)
 - [Requirements](#requirements)
 - [Development Workflow](#development-workflow)
 - [Build](#build)
@@ -12,6 +13,16 @@
 
 - [Detailed CI and build process docs](../ci)
 
+## Pull Requests
+
+Please link to the issue each PR solves.
+If there is no existing issue, please first create one unless the fix is minor.
+
+Please make sure the base of your PR is the master branch. We keep the GitHub
+default branch the latest release branch to avoid confusion as the
+documentation is on GitHub and we don't want users to see docs on unreleased
+features.
+
 ## Requirements
 
 Please refer to [VS Code's prerequisites](https://github.com/Microsoft/vscode/wiki/How-to-Contribute#prerequisites).