Add a document somewhere saying how to experiment with Zcash

[str4d]

e.g. if someone wants to define a new transfer protocol (like Orchard, or zkChannels), or a new detection protocol (requiring Unified Address support), there should be a nice page somewhere that people can go to, that summarises things like:

• What kinds of "values" (constants, typecodes etc.) can just be picked arbitrarily.
• What kinds of "values" MUST NOT be picked arbitrarily, and must go through a ZIP process.
• Suggestions about what parts of the existing protocol could be repurposed "safely" based on the current assumptions (e.g. encCiphertext is assumed to be indistinguishable from random unless you are an intended recipient, so for dummy notes it could be repurposed as not a ChaCha20Poly1305 ciphertext).
• Links to the ZIP process, and maybe a summary of how to go about proposing edits (e.g. to request a new UA typecode).
• The existence of testnet-in-a-box, and links to usage info.

This might go on zips.z.cash, or in an mdBook somewhere.

[str4d]

Other things:

• How to use TZEs for experimenting with other pools.
• How to extend the transaction format to add extra fields (e.g. detection key stuff)