Include a known-good llvm revision

[stephenatwork]

Because CppSharp requires a trunk version of llvm it would be nice to keep the an up to date known-good revision number in the version getting started docs.

[mydogisbox]

In the couple of months I've been playing with it, I haven't had to use anything other than the latest version of trunk. For this, my preference would be that a particular commit would note that it's time to move to the latest version of the llvm trunk. Additionally, note that there are binaries for CppSharp on the main page, which I assume don't depend on llvm (although I haven't tried).

[stephenatwork]

OK, maybe this isn't a big deal - I made a mistake in using an old version of CppSharp and had to patch a few things to make it work with the latest llvm.

[tritao]

I agree this will be useful, actually someone already had requested this before.

The next time I update LLVM/Clang, I will include the SVN revision I used.

[tritao]

Done.