Explain meta-schema status and support

Every time we fix a bug in place someone complains, so explain somewhere that we do that, what it means, and that it's on purpose. Also say something about what sort of changes would not be fixed in place.

TL;DR: If the meta-schema contradicts the spec, and the fix will not cause problems with users who were complying with reasonable interpretations the spec, then we'll fix it in place. Examples:

• Leaving out a keyword. Adding it in will only cause problems if someone was using the keyword for something else. Most users would prefer the meta-schema to detect improper use
• Egregiously wrong defaults, like the string "true" instead of the boolean true. Defaults are not intended to be automatically written into instances, but if the default is the wrong type then anyone who attempts to use it will have problems.

I can't come up with something that we wouldn't fix in place, as the more dramatically breaking things would probably be caught as soon as people start using it. In which case it's better to fix even a dramatic thing in place. But let's say something about the possibility so people don't completely freak out thinking that we'll just arbitrarily mess with it.

Start by locating the website's existing documentation about meta-schema status and support. Add an explanation of intentional in-place fixes, including omitted keywords and incorrectly typed defaults, and describe the limits or considerations for changes that could break users. Done means the policy is published in an appropriate documentation location and clearly addresses the concerns in this issue.