Merge pull request #393 from pesho/contributing-yarn

Add CONTRIBUTING.md including Yarn version update policy

Author: pesho

CONTRIBUTING.md

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+# Version Updates
+
+New **Node.js** releases are released as soon as possible.
+
+New **NPM** releases are not tracked. We simply use the NPM version bundled in the corresponding Node.js release.
+
+**Yarn** is updated to the latest version only when there is a new Node.js SemVer-minor release, and it's updated only in the branch with the new release, preferably in the same PR. The `update.sh` script does this automatically when invoked with a specific branch, e.g. `./update.sh 6.10`.