README.md: Fix grammar, new *Upgrading* section
README.md: Fix grammar, new *Upgrading* section

This commit should resolve #261 where @wking suggested to update
`README.md`.

A few grammatical errors were fixed.

Sections were introduced (`Getting started`, `Upgrading`, `Contact`).

A new step to *Getting started* section was introduced (cloning the
repository).  Some notes were added.

A new section, *Upgrading*, was added (this directly relates to the
suggestion in #261).

wking/swc-amy,vahtras/amy,swcarpentry/amy,pbanaszkiewicz/amy,pbanaszkiewicz/amy,vahtras/amy,pbanaszkiewicz/amy,wking/swc-amy,shapiromatron/amy,vahtras/amy,swcarpentry/amy,shapiromatron/amy,swcarpentry/amy,wking/swc-amy,shapiromatron/amy,wking/swc-amy