Update README.markdown

You need to change to the root of your repo for a git-hg push to work.

Also, some question/answers to be added to readme:

* how do I get a list of destinations? Do you mean hg destinations?
* where are hg destinations configured in git-hg?

(I'm proficient in git but not hg, so I don't know these answers. Nor would other git-centric users).

Also, pushing seems to take for ever on large repos, is this to be expected?