- Recommend an explicit directory name for the clone, to avoid cloning
  into ~/atheme/ if the user runs the `git clone` operation in their
  home directory. Atheme defaults to installing to ~/atheme/, and you
  cannot install Atheme to its source directory. [1]

- Recommend the use of the `--recursive` option to `git clone` in
  `README.md`, as it was already recommended in `GIT-Access.txt`. [1]

- Provide alternative command sequences for people who have very old
  versions of git, which may not even support the `--init` option of
  `git submodule update`, let alone the `--recursive` option of
  `git clone`.

- Quote a filesystem path.

- Tidy up the more information section in the bottom of `README.md`.

[1] Suggested by GitHub user @PeGaSuS-Coder in PR #764

Closes #764