README.md: period at end of sentences
README.md: fix spelling mistakes

Mistakes found by aspell list < README.md. Some words were not fixed
because they weren't English words in the first place (like
'downloadfile', which is a getbymail command itself).
README.md: fix markup for headers
README.md: fix general markup issues

This fixes an item list and code snippets.
README.md: fix -s in third-person usage of verbs
README.md: fix small typo