Add .yarnrc.yml

This was created when first running `yarn install` using yarn version 4.

- `approvedGitRepositories`[1] has been set to allow installing from any
  git repo.

- `enableScripts`[2] has been set to true despite the default value
  being false. I can only guess this is because it's retaining the
  behaviour from yarn version 1 that we were using previously.

- `nodeLinker`[3] has been set to `node-modules` as that was the default
  behaviour in yarn version 1 that we were using previously.

[1]: https://yarnpkg.com/configuration/yarnrc#approvedGitRepositories
[2]: https://yarnpkg.com/configuration/yarnrc#enableScripts
[3]: https://yarnpkg.com/configuration/yarnrc#nodeLinker

Author: chrisroos

.yarnrc.yml

@@ -0,0 +1,6 @@
+approvedGitRepositories:
+  - "**"
+
+enableScripts: true
+
+nodeLinker: node-modules