docs: warn maintainers about pinned Twisted version

...that was introduced in f1b3e67.

Move to version 0.1.1

Add a modification notice

Since we're about to officially distribute the source, add a
modification notice to comply with the Apache licensing requirements.

Make output bucket brigade thread safe

Rewrote Alex Bligh's pull request so that it matches the coding style of
this project and it should now compile with MSVC, which is not C99
compliant (i.e., doesn't support mixing of statements and declarations).