Document the autoconf process.

[jlm]

Update the README to describe how to run autoconf to generate the ./configure script.

[stappersg]

It is better as #258 and should be improved:

• Split in "Fear mongering you need Docker" and "Updating the README"
• Add commit messages to the commit summary
• Add sign off by

[stappersg]

And I don't like the white space change.

@jlm if the white space change is important for you, document the why in the commit message. (Preferable in a separate merge request.)

[robbat2]

The whitespace change is fine with me, as we could probably change this to markdown in future.

However it does need a DCO-style signed-off-by line on the PR & commit.