make docs - lein not found

[aaronmarkham]

Lein needs to be added to the docs build instructions.

If you're stuck on this error, do this:

wget https://raw.githubusercontent.com/technomancy/leiningen/stable/bin/lein
chmod 775 lein
sudo cp lein /usr/local/bin

Or some directory that is in your $PATH. Then re-run, and you should be good to go.

I'll submit a PR to add this to the instructions.

[aaronmarkham]

I'm going to wait until this PR is merged: #11590
Then I can use the installation script I made for the Ubuntu deps as a base rather than starting from scratch.

[apeforest]

@sandeep-krishnamurthy Please label this Build, Documentation