Add CONTRIBUTING.md with instructions to bump submodules

CONTRIBUTING.md

@@ -0,0 +1,57 @@
+Contributing to Rocket Chip
+=====================
+
+Thank you for your interest in contributing to Rocket Chip!
+
+## Table of Contents
+
++ [Bumping Submodules](#bumping)
+
+### <a name="bumping"></a> Bumping Submodules
+
+Several projects are managed as git submodules as well as [Wit](https://github.com/sifive/wit) dependencies.
+
+### When to bump
+
+Most projects will be bumped by developers as needed; however,
+sometimes users may wish to speed up the bumping process.
+For more stable projects like Chisel 3 and FIRRTL,
+please only bump to stable branches as defined by the specific subproject.
+As of March 2020 these branches are `3.2.x` in Chisel 3 and `1.2.x` in FIRRTL.
+
+### How to bump
+
+1. Bump the Git submodule
+
+```bash
+# Check out a branch (starting at rocket-chip root
+git checkout -b my-bumping-branch
+
+# Check out the commit of the submodule you wish to bump to
+cd <submodule>
+git checkout <commit>
+cd ..
+```
+
+2. Bump the Wit submodule
+
+Update the `commit` field for the specific submodule in `wit-manifest.json`.
+You can do this by simply editing the file in your text editor of choice.
+
+**Tip** `git -C <submodule> rev-parse HEAD` will give you the commit hash
+
+3. Commit the changes
+
+```bash
+# Add and commit the submodule
+git add <submodule>
+git add wit-manifest.json
+git commit -m "<meaningful message about bumping>"
+```
+
+If you are bumping `Chisel 3` or `FIRRTL`, it is ideal to include some notes about
+major feature or performance improvements in your commit message.
+
+4. Open a Pull Request on Github
+
+Please see the Github documentation for [Pull Requests](https://help.github.com/en/github/collaborating-with-issues-and-pull-requests/proposing-changes-to-your-work-with-pull-requests)

README.md

@@ -650,7 +650,9 @@ Further information about GDB debugging is available [here](https://sourceware.o
 
 ## <a name="contributors"></a> Contributors
 
-Can be found [here](https://github.com/ucb-bar/rocket-chip/graphs/contributors).
+Contributing guidelines can be found in [CONTRIBUTING.md](CONTRIBUTING.md).
+
+A list of contributors can be found [here](https://github.com/chipsalliance/rocket-chip/graphs/contributors).
 
 ## <a name="attribution"></a> Attribution