INSTALL file

Hello,

while working with Google Code-In students, there is one task: "clone
PostgreSQL from git repository, and build from source".

This brought up an interesting problem: the README refers to an "INSTALL"
file, which is present in packages, but not in the source repo. This is
very confusing for beginners, and should be avoided.

There are a couple options to fix this:

1. Update the README, and remove the "INSTALL" reference
2. Create a permanent INSTALL file, and possibly overwrite it during
packaging
3. Add different INSTALL files, based on the platform (Linux, BSD,
Windows, Source)

Regards,

--
Andreas 'ads' Scherbaum
German PostgreSQL User Group
European PostgreSQL User Group - Board of Directors
Volunteer Regional Contact, Germany - PostgreSQL Project