docs: update README testing section and add fetch vs pull explanation

[Iamrodos]

Two changes:

1. Testing section - updated to reflect that tests now exist
2. Fetch vs pull FAQ - explains why github-backup uses fetch (avoids merge conflicts), how to view updated files, and suggests --bare for those who don't need a working directory

The fetch/pull question comes up regularly (#196, #269, #109) - documenting it allows those issues to be closed.

No release required - documentation only, can go out with the next release.