Updated terminology wiki page

[echeran]

In order to organize the terms we've used (see #30), I've created a draft updated page at https://github.com/echeran/message-format-wg/wiki/Glossary-&-Resources.

Because Github does not support creating pull requests for the repository-style access it allows for wiki pages, this issue is the closest substitute for a PR. The git-access for the proposed changes is at https://github.com/echeran/message-format-wg.wiki.git

[echeran]

I am folding this issue into PR #80 since we decided in the meeting to stop using wiki pages in favor of markdown files in the repo.