Rewrite README.md sentence by sentence (closes #190)

README.md is rewritten from scratch against the rules on issue 190 and
checked statement by statement against the code on next. It leads with
what the program is and how to use it, puts every endpoint, command,
numeric reply, mode and setting in reference sections, and ends with
development and testing. Where the old text disagreed with the code,
the code decides; the PR body lists each case. The hashcash examples
are real 20-bit stamps. REPO_POLICIES.md and CONVENTIONS.md are copies
of shared documents and are untouched.

Model: fable-5-1

schema/README.md

@@ -43,9 +43,9 @@ IRC:  :alice!alice@* PRIVMSG #general :hello world
 JSON: {"command": "PRIVMSG", "from": "alice", "to": "#general", "body": ["hello world"]}
 ```
 
-A numeric reply has its name in `command` and its number in `code`. This `353`
-reply lists the members of `#general`. The JSON again leaves out `id`, `ts` and
-`meta`:
+In the JSON, a numeric reply has its name in `command` and its number in `code`.
+This `353` reply lists the members of `#general`. The JSON again leaves out
+`id`, `ts` and `meta`:
 
 ```
 {"command": "RPL_NAMREPLY", "code": 353, "from": "neoirc", "to": "alice",