Write Design doc for Webassembly.sh File System

[torch2424]

For Example:

• where are commands stored from wapm? @syrusakbary Suggests /bin.
• Where are files uploaded? @syrusakbary suggests current directory
• How are custom modules run? @syrusakbary suggests ./modulename.wasm
• How are files downloaded back to the host?
• What is the command resolution?
• How are commands versioned?
• Are files overwritten? If not, how are are they renamed?
• And much more!

[torch2424]

This was already implemented, but still need to document, cc @syrusakbary