Adding PGInstaller to the Downloads section

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!