Add documentation on branching old versions (#8647)

Author: helin24

docs/building.md

@@ -60,3 +60,42 @@ In order to update or add a new pre-req:
 ```shell
 $ gsutil cp <path-to-archive> gs://flutter_infra_release/flutter/intellij/
 ```
+
+## Branching to manage IDEA platform versions
+
+### Strategy
+
+(The below is a summary of an internal doc on our strategy for version management.)
+
+The main branch intends to support multiple IDEA platform versions (e.g. currently, 2025.1 to 2025.3). This simplifies
+development as only one plugin is typically built and released from main.
+
+However, there may be times when we need different builds for different versions of the IDEA platform; for example, if we introduce a
+feature that requires platform version 2025.2 and above, but we still want to release the plugin for version 2025.1. In this case, we plan
+to use just-in-time version branching to create a "maintenance" branch for an older platform version. We may also start a maintenance branch
+because we no longer find it necessary to release new plugins for an outdated platform.
+
+### Branching procedure
+
+This is an example of the procedure:
+
+1. Notice that a change that breaks previous versions has been introduced.
+2. Find the last commit that was included in the last plugin release that covered the old platform version (you may need to refer
+   to https://plugins.jetbrains.com/plugin/9212-flutter/edit/versions/stable) along with commit history.
+3. At this commit, create a branch for the old platform version(s) where the breaking change cannot be applied, e.g. `platform-251` to
+   indicate 2025.1. To create a branch on an old commit on GitHub, find the commit, click the `<>` button ("browser repository at this
+   point"), click on the tree icon at the top left, and then type in the new branch name.
+
+### Releasing from a maintenance branch
+
+In many cases, we may not ever build a release from a maintenance branch, and we will just keep it for history. If we do want to release
+from a maintenance branch, do the following:
+
+1. Cherry-pick commits to the maintenance branch (e.g. `platform-251`) that should be released for the old platform version.
+2. Check out the maintenance branch and build according to the instructions above.
+
+### Existing maintenance branches
+
+| Old IDEA platform version (branch)                                                     | Last released Flutter Plugin version |
+|----------------------------------------------------------------------------------------|--------------------------------------|
+| 2024.3 ([platform-243](https://github.com/flutter/flutter-intellij/tree/platform-243)) | 88.1.0                               |