Adding PGInstaller to the Downloads section

Hi,

Please find attached a patch to add PGInstaller to the Downloads section of
postgresql.org.

PGInstaller offers an easy way to install PostgreSQL. We have found this to
be crucial to our customers who are used to GUI-based, click-n-point
installers on Windows and at times on macOS. PGInstaller has evolved from
2UDA, a package that 2ndQuadrant has been working on for more than 3 years.

2ndQuadrant is committed to developing and actively maintaining PGInstaller
for PostgreSQL versions 9.5 and above. We have been releasing, and will
continue to release, updates in sync with the PostgreSQL community, most
recently announcing the minor release on 9-Aug. PGInstaller has also kept
up with the Beta releases for PostgreSQL 11.

Thanks!

--
Umair Shahid
Head of Marketing & Products
2ndQuadrant - The team of diligent PostgreSQL experts
https://www.2ndQuadrant.com/