docs(community build): update the community build readme

Author: ckipp01

community-build/README.md

@@ -1,22 +1,75 @@
 # Scala 3 Community Build
 
-This project contains tests to build and test a corpus of open sources Scala projects against the latest version of Scala 3.
+This project contains tests to build and test a corpus of open sources Scala
+projects against the latest version of Scala 3.
 
 ## Running it locally
 
-To run the community build on a local machine, first fetch all the git submodules with `git submodule update --init` and run `sbt community-build/test` from the root of the dotty repo.
+To run the community build on a local machine, first fetch all the git
+submodules with `git submodule update --init` and run `sbt community-build/test`
+from the root of the dotty repo.
 
-To run a single project, you can use the usual syntax for running a single JUnit test, for example `community-build/testOnly -- *shapeless`
+To run a single project, you can use the usual syntax for running a single JUnit
+test, for example `community-build/testOnly -- *shapeless`
+
+In CI the community build is split up into 3 seperate groups: A, B, and C. To
+run one specific build you can also use the same JUnit syntax as above targeting
+the individual group. For example:
+
+```
+sbt "community-build/testOnly dotty.communitybuild.CommunityBuildTestA"
+```
 
 ## Adding your project
 
-To add your project to the community build you can follow these steps:
+The community build is able to handle both Mill and sbt projects. To add your
+project to the community build you can follow these steps:
 
-1. Get your project to compile with Scala 3. Instructions can be found on the [scala3-example-project](https://github.com/lampepfl/scala3-example-project).
-   See the submodules in [community-projects](https://github.com/lampepfl/dotty/tree/master/community-build/community-projects/) for examples of projects that compile with Scala 3.
+1. Ensure your project is compiling with Scala 3. If you need help make sure to
+   check out the [Scala 3 Migration
+   Guide](https://docs.scala-lang.org/scala3/guides/migration/compatibility-intro.html).
+   You can see the submodules in
+   [community-projects](https://github.com/lampepfl/dotty/tree/main/community-build/community-projects/)
+   for examples of projects that compile with Scala 3.
 
 2. Open a PR against this repo that:
      - Adds your project as a new git submodule
        - `git submodule add https://github.com/dotty-staging/XYZ.git community-build/community-projects/XYZ`
-     - Add the project to [projects.scala](https://github.com/lampepfl/dotty/blob/master/community-build/src/scala/dotty/communitybuild/projects.scala)
-     - Adds a test in [CommunityBuildTest.scala](https://github.com/lampepfl/dotty/blob/master/community-build/test/scala/dotty/communitybuild/CommunityBuildTest.scala)
+     - Add the project to [projects.scala](https://github.com/lampepfl/dotty/blob/main/community-build/src/scala/dotty/communitybuild/projects.scala)
+     - Adds a test in [CommunityBuildTest.scala](https://github.com/lampepfl/dotty/blob/main/community-build/test/scala/dotty/communitybuild/CommunityBuildTest.scala)
+
+3. Once the CI is green, someone from the Dotty team will fork your repo and add
+   it to [dotty-staging](https://github.com/dotty-staging). This enables us to
+   make changes to your fork if necessary to keep the community build running
+   smoothly.
+
+4. Once the fork is created, please update your PR to point to this new fork
+   instead of your repo.
+
+## Updating a project
+
+The projects included in the community build are all forked and located in
+[dotty-staging](https://github.com/dotty-staging). When something needs to be
+bumped the process is as follows:
+
+1. Fork the dotty staging repo and sync it with the upstream project.
+
+2. Once you've verified that the tests are all passing you can then either
+   request in your PR that the dotty-staging fork be synced or in the
+   [scala-contributors](https://discord.com/channels/632150470000902164/632628489719382036)
+   discord channel.
+
+### Some helpful tips
+
+- If you're unfamiliar with Git Submodules you can find a nice guide to get
+    familiar with them [here](https://git-scm.com/book/en/v2/Git-Tools-Submodules).
+- Keep in mind that many projects are interrelated. So when you bump one that
+    change may cascade through multiple different projects causing you to have
+    to bump multiple. Plan accordingly and at times it's best to pin it to a
+    stable release version, especially if it's a root library that many others
+    in the community build are relying on.
+
+## Looking for the "unmanaged" Scala 3 community build?
+
+You can find this [here](https://github.com/VirtusLab/community-build3).
+