Store should be immutable

[zkochan]

Added a section about store immutability.

I see two possibilities to achieve it: copying and hard linking. However, hard linking would allow editing... Howeverm, it is a huge space saver, so I really would love to keep the option of linking

[billiegoose]

@zkochan There's more than two possibilities! Here's all the one's I've thought of till now:

• copying
• hard linking
• filesystem emulation at the OS level (i.e. FUSE)
• filesystem manipulation at the node level (hijack the 'fs' module)
• require manipulation at the node level (hijack the 'require' module)

There are various pros and cons that I haven't mentally sorted out yet. Some of them are install time manipulations (hard linking) while others require a runtime modification (hijacking the 'require' module). I'm leaning toward trying hijacking the 'require' module myself, but I simply have not had time.

p.s. Hard-linking only allows edits if the file permissions allow edits. My suggestion would be to have a separate user 'node' be the store owner and allow others only read+execute permissions.