README: improve overall quality

[trueNAHO]

• README: add empty lines for consistent paragraph formatting
• README: capitalize bullet points consistently
• README: punctuate bullet points consistently
• README: format code blocks with bash instead of shell
  • The GitHub previewer has better color support for bash code blocks than for shell code blocks`.
• README: remove initial code block indentation
• README: code format commands and command names

[newren]

The commits generally look good. I like the split into separate commits, and you describe each change well.

While changing "filter-branch" to "filter-branch" is generally good and correct, changing "git filter-branch" to "git filter-branch" is not correct; the backtick should start before "git" in that case. I caught at least one example of that problem.

However, there's one big problem here -- I need to you to use your real name and real email in the author & committer, and include a Signed-off-by line (https://github.com/git/git/blob/master/Documentation/SubmittingPatches#L351). Can you do that? If so (and if you fix the backtick issue I mentioned above), I'd be happy to merge the changes.