Set general requirements and process for including wallets

Preview: https://github.com/bitcoin/bitcoin.org/tree/walletguidelines#wallets

As suggested in #663, I am writing a list of requirements and process for inclusion used thus far. As a matter of fact I have been slowly accumulating this list from developers comments or real issues when testing wallets. Recent wallets were tested with these "standards".

This is in no way a comprehensive list and I still think the first point in the list (sufficient developers and users feedback) is the most important one. In an ideal world bitcoin.org would have someone dedicated to auditing wallets' code and all (which is obviously out of reach for now).

Additionally, this list is meant to change, similarly to scores which evolved with the new unique security models of multisig and hardware wallets. To the best of my knowledge, I've tried to observe very common, simple or non-obtrusive security practices being adopted by all wallets.

Suggestions are welcome if you want to improve this process and involve with the maintenance of this page.

Note: Xapo is the last web wallet to not support HSTS and KnC/Hive for Mac are the last wallets to not rotate change addresses. It may be good to consider these becoming a requirement soon.