Make release-process.md followable verbatim

The process outlined in the current release-process.md uses pushd and popd, to dip into and return from directories. It starts in the directory that contains the gitian-builder, gitian.sigs, and bitcoin directories. It then changes directory to the gitian-builder directory, builds, puts the signature into gitian.sigs, and the build outputs zipped up in the gitian-builder directory. Then, it repackages the builds into stand-alone files, still within gitian-builder. Once that's done, the time comes to commit the signatures and then, once enough people have signed, to repackage the gitian zips. At this point, the document appears to assume you're already back in the initial directory, the one with the gitian zips (which are actually in gitian-builder), gitian.sigs and bitcoin directories. This edit addresses these issues, by putting the gitian zips in that top directory, and using that directory as the base camp, so to speak, from which the various directories are dipped into and returned from throughout the process.