This PR adds some documentation that:
* Describes who the audience for the `docs/`, `docs/dev/` and `docs/admin/` directories are, as well as Synapse's wiki page.
* Stresses that we'd like all documentation to be down in markdown.
This PR makes several changes to the `./scripts-dev/lint.sh` script, which lints the codebase with a number of tools:
* Adds usage information, with `-h` flag to show it. Otherwise it will show when providing an unknown flag.
* Adds option `-d` which will check both staged and unstaged files that have changed since the last commit and add them to the list of files to lint.
- Note that only files without an extension, or with a `.py` extension will be allowed. This prevents editing bash scripts causing the linters to break on non-python files.
* Improves the print-out of which files/directories are being linted.
