Build `lerna` development dependency in/before bootstrapping

[GuiltyDolphin]

The readme currently states (in contributing):

After you clone the code:

#+begin_src shell
# Bootstrap the Project
yarn run bootstrap

# Run Unit Tests
yarn test

# Play with the Website
cd www
yarn run develop
#+end_src

However, running yarn run bootstrap at this stage just yields an error informing us that the lerna command is not found.

We should install lerna as part of (or before) the bootstrapping process, so that this error doesn't arise.