Clarify scope in README

[gjermundgaraba]

It would be very useful for contributors and readers if the scope of the book is made much clearer in the README, and that it links readers to docs or other resources that might include topics that are not in scope of the book.

This was also discussed briefly on the community call and in Discord.

This is also cross-posted to the CosmWasm/docs repo as they are connected and are relevant for both repos: https://github.com/CosmWasm/docs/issues/440