README.md: Fix typos, grammar, add clarifications, remove unnecessary prerequisite

* marmalade and melpa instructions were the wrong way round
* minibuffer gets stuck if firefox is not already running
* `C-c o` only works in an org-mode buffer
* prerequisite is not needed any more
Merge pull request #45 from greenrd/master

README.md: Fix typos, grammar, add clarifications, remove prerequisite