Add changes section to Tor bridge descriptors specification

atagar suggest on tor-project@ to add a changes section to our new Tor bridge descriptors specification:

One quick thing: when we next change sanitization would you mind adding a 'changes' section with the date of the change and what the modification was? I keep an eye on the tor-spec git changes to know when the dir-spec changes but a webpage like this is harder to track. A list to answer the question of "Last time I checked this was August. Has anything changed?" would be handy.

Sounds useful to me. Let's start with such a changes section now, so that it doesn't appear out of the blue whenever we next make a change. I dug through Git history and found the following changes affecting @type annotations.

Dates are Git's "committer dates" rather than "author dates". In the future we could switch to release dates as an even better approximation of when changes became effective, but past changes were usually deployed whenever they arrived in master. In the end the exact timing doesn't matter as much, though.

• May 30, 2012: "@type" annotations were first added to sanitized descriptors to indicate descriptor type and version: "@type bridge-server-descriptor 1.0", "@type bridge-extra-info 1.0", and "@type bridge-network-status 1.0".
• June 29, 2012: "@type bridge-extra-info 1.1" added sanitized "transport" lines.
• February 1, 2013: "@type bridge-extra-info 1.2" was supposed to indicate added "ntor-onion-key" lines, but those changes only affected bridge server descriptors, not extra-info descriptors. So, nothing has changed as compared to "@type bridge-extra-info 1.1", and there may be "@type bridge-server-descriptor 1.0" descriptors with and without those lines.
• June 19, 2015: "@type bridge-server-descriptor 1.1" and "@type bridge-extra-info 1.3" added "master-key-ed25519" and "router-digest-sha256" lines to descriptors published by bridges using an ed25519 master key.
• September 18, 2016: "@type bridge-server-descriptor 1.2" and "@type bridge-network-status 1.1" introduced sanitized TCP ports.
• July 10, 2017: "@type bridge-network-status 1.2" introduced the "fingerprint" line, containing the fingerprint of the bridge authority which produced the document, to the header.

Please, somebody (probably atagar or iwakeh), review this list. If it looks good, I'll put it on the page early next week.