Extend README for Adblock Browser development

[rjeschke]

As we are not on the development version of Fennec, out build setup might lag behind the one described ​here. This makes it hard to just check out the project and build it.

The solution is to put a more detailed build setup into our README (i.e. copying and modifying parts of Mozilla's instructions).

Things we need to have in our README would be:

• SDK and NDK versions and special cases (like e.g. downgrading the support library, and how to do this)
• other build steps (e.g. ensure_dependencies.py)
• complete mozconfig (with all our special flags, maybe we should check in a mozconfig.adblockbrowser.template for that case, also)
• instructions for multi-locale builds

See also #2791 for possible release build instructions to append to the README

[fhd]

While I agree that we should eventually stop referring to Fennec's build instructions, for now I see an easier option: We link to the specifically relevant version of those instructions in our README. We could use web.archive.org links e.g.

Still, we can keep this issue around since I believe we do eventually want to have the build instructions in the README here. I don't think they belong in a wiki, that won't be in sync with older versions then.

[Mailkov]

When I talked on IRC with rjeschke about this problem four weeks ago,
I had the impression of being considered crazy!

I'm happy that this issue was opened. (better late than never)

[jwangenheim]

Even if this is a quite old issue, it maybe still worth taking a look at. After we recently had some confusions about it, we should probably also add to the README that Artifact Builds do not work for ABB.

Diego already added the new link to the build instructions (​https://hg.adblockplus.org/adblockbrowser/rev/6c07df33154d) as the one in the description of the ticket is not valid anymore.