Add yarn to Contributing: Development Dependencies

[waciumawanjohi]

Bug description:

Yarn not mentioned in dev dependencies.

Steps to reproduce:

Please provide a set of steps to reproduce the behavior (example):

1. Run make lint

Expected behavior:

As a contributor, when I've installed the development dependencies, I should be able to run the make commands.

Actual behavior:

/Library/Developer/CommandLineTools/usr/bin/make -C site lint
yarn install
make[1]: yarn: No such file or directory
make[1]: *** [dev-dependencies] Error 1
make: *** [lint] Error 2

[squeedee]

Can we block this on the site migration (#436), and add it to the site repo after the migration?