Add section about goods assets library

[zakarumych]

It's the first time I write something like this, so just point me the issue ;)

@ozkriff asked to link it here #139

[ozkriff]

Nice! Just a few style nitpicks:

1. Could you insert hard-wraps at 80 chars?
2. There's a lot of short paragraphs, could you either merge some of them or convert to an explicit list, please?

[ozkriff]

👍 (tweaked fmt a bit)

[ozkriff]

Merged, thanks for the PR!