Add disk space considerations to Postgres porting instructions

[schildbach]

I think on the SQLite→Postgres porting instructions some considerations about disk space usage should be added.

My homeserver.db was 1.6G and is now almost double in size after migrating to postgres:

# du -sh /var/lib/postgresql/
3.1G    /var/lib/postgresql/

This means that for following the instructions step-by-step you need <size of homeserver.db> * 4 space on your harddrive:

• homeserver.db
• homeserver.db.snapshot
• /var/lib/postgresql/

[HarHarLinks]

My migration resulted in the following sizes (+24%):
4.9G homeserver.db
6.1G /var/lib/postgresql