Tidy up landing page on GitHub (README.md)

[mlazdans]

Let's keep landing page a bit more clean? I suggest:

• Moving build instructions to Wiki or to separate file, say BUILDING.md
• Add some quick information:
  • What is this project, current version, etc
  • What PHP / FB / Arch combinations are supported
  • How it differs from PDO_Firebird
  • ...
• ...

[MartinKoeditz]

Agreed.