docs: update njs build instructions

[JTorreG]

Update the Enabling njs steps to include changing branches to the latest supported version

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[ac000]

njs has moved to GitHub

[callahad]

Looks reasonable. Two small suggestions at your option.

[callahad]

Seconding @ac000's suggestion of removing mentions of mercurial given the mailing list announcement he pointed to.

[JTorreG]

removed the mercurial steps
changed the git steps to

 $ git clone https://github.com/nginx/njs.git
 $ cd njs
 $ git checkout -b 0.8.2 0.8.2