Add some links for sandstorm-pkgdef.capnp

[mitar]

Instructions in this README explain to edit sandstorm-pkgdef.capnp and just read comments. But comments are not enough really to be able to edit the file well. Some link to some documentation with more information would be good.

Unclear questions:

• like, why I have to rename "New Instance" command text, where is that even shown?
• how sourceMap work and what are those ".meteor-spk/deps" and ".meteor-spk/bundle", where does this come from?
• similarly, alwaysInclude = [ "." ]

[mitar]

Also, it is unclear what I have to edit just to be able to test locally the app and what I have to edit for publishing later on. First I would even like to just run it so that I can port the code itself. I do not believe it will just work.