add a note about Effing Package Management and elaborate on prototype

[milancurcic]

This PR:

• Adds a clarification about the other fpm
• Expands the note on fpm being an early prototype

Fixes #90

[everythingfunctional]

Minor wording change suggestion, but other than that this looks good. Thanks for tackling this one.

[certik]

I would also add at the end:

As the prototype matures and we enter production, we will do our best to stay backwards compatible.

(Otherwise looks good, thanks!)

[milancurcic]

Thank you, both. Merging.