Document how to install / run web-ext from source

[kumar303]

needs explicit instructions in the README

[Mte90]

web-ext run -s /path/extension/ --firefox-binary=/path/of/firefox
web-ext build -s /path/extension/

I used this to run and build the extension

[kumar303]

I meant for how to run install the command from source. I think it's something like:

git clone ...
npm install
npm run build
npm link
web-ext ...