doc: readme: removed all mentions of PUC-Rio Lua and fixed styling and grammatical issues.

* Removed mentions of PUC-Rio Lua, or hints that this module may work
  with it.
* Clarified that this module only supports LuaJIT 2.x starting with
  v0.10.16.
* Clarified that the terms "Lua" and "LuaJIT" are used interchangeably
  in this document.
* Consistently capitalized "Nginx".
* Many styling and grammatical fixes.

Author: thibaultcha