README: Improve section order
README: Improve section order

New users should see the most relevant sections first. In particular,

- Examples
- Installation
- Usage

dockcross/dockcross,dockcross/dockcross,dockcross/dockcross,dockcross/dockcross,thewtex/cross-compilers,thewtex/cross-compilers,thewtex/cross-compilers,thewtex/cross-compilers