Instructions in the README for building the BigCouch merge

[nolanlawson]

I've been trying to wrap my head around how to get an alpha version of CouchDB 2.0 up and running. I tried following instructions from @wohali here but got stuck due to incompatible Spidermonkey and Erlang versions.

In the past I've always used build-couchdb with minimal fuss, but I'm not sure how to use it to build the bigcouch branch. I tried a few different strategies for pulling in the 1843-feature-bigcouch from http://git-wip-us.apache.org/repos/asf/couchdb.git, but with no success – the git= option seems to either not work or I'm not understanding it, and manually changing the git branch causes the build to exit early with no error message. Advice?