Document Tessel 2 memory and storage

[reconbot]

• How does it work?
  • How much space is on the tessel?
  • Where is it safe to write?
  • Is there temporary storage?
• Can I use USB storage?
  • What filesystem does it need to be?

[reconbot]

Similar stuff to http://tessel.github.io/t2-start/modules/storage.html

[reconbot]

Also the storage can be at sda, sda1, sdb, sdb1 etc, depending on how its partitioned and formatted.