gitian: explicitly enumerate manual steps prerequisites for gitian-build-sh

[Sjors]

At this point you have two options, you can either use the automated script (found in https://github.com/bitcoin/bitcoin/blob/master/contrib/gitian-build.sh, only works in Debian/Ubuntu) or you could manually do everything by following this guide.

Lies! :-) You always need to do some things manually, or choose to produce an incomplete set of binaries. This should be clarified.

1. MacOS build will be skipped, unless you (insert link to instructions, see gitian: simplified OSX instructions #13)

[Sjors]

It's actually only 1 thing, so should be easy to clarify.

[Sjors]

Fixed via #25.