Add installation guide to README

This pull request introduces an installation guide to the README file. To remain consistent, I based the guide on [ReactiveCocoa's one](https://github.com/ReactiveCocoa/ReactiveCocoa#system-requirements) which is also developed by GitHub, [I believe](https://github.com/blog/1107-reactivecocoa-for-a-better-world).

Sinc the 6th point of the installation guide instructs to add an `-ObjC` linker flag, this pull request closes #93.

#### Changes

- Removed the "Getting Started" section
- Added "System Requirements" and "Installing Mantle" sections
- Improved formatting in some places (hard-wrapped at 80 chars to match the rest of the README)