build: vendor yarn using the .yarnrc yarn-path value

Rather than enforcing yarn versioning using `package.json`'s
engines value.  We can utilize yarn's `yarn-path` value to
ensure that the version of yarn used at execution time is
consistent for everyone who uses our repo.

.yarn/README.md

@@ -0,0 +1,13 @@
+# Yarn Vendoring
+We utilize Yarn's `yarn-path` configuration in a shared `.yarnrc` file to enforce
+everyone using the same version of Yarn.  Yarn checks the `.yarnrc` file to
+determine if yarn should delegate the command to a vendored version at the
+provided path.
+
+## How to update
+To update to the latest version of Yarn as our vendored version:
+- Run this command
+```sh
+yarn policies set-version latest
+```
+- Remove the previous version