Move postgresql DB to larger disk, document in INSTALL

[dginev]

Method described here and here

The DB is at 21GB for about 100K converted LaTeXML jobs, so it will be at an estimated 200 GB for the final state. The root drive won't fit that, so I'll move it after I fix #9 and update, so that I can also stop postgres for the move.

[dginev]

Actually, it's 21GB for 360K converted jobs, so the estimate total will be 3 times that, or 63GB. That fits on disk! The move can be postponed until we get near the limit, closing here.