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.  This is the first
step in this wider vendoring process.  We will use this same
vendoring mechanism for CI after
bazel-contrib/rules_nodejs#1569 lands

.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