convert README to GitHub-flavored Markdown

[salim-b]

A GFM-formatted README offers the following advantages:

• Rendered beautifully on github.com, thus offers better readability for visitors (and in the end dramatically increases the chance people actually read it 😜). Internal links (i.e. anchor links) further simplify navigation in this quite extensive README.

• GFM is a well-defined format which is supported by a wide range of tools including Pandoc. This means you can now convert the README to all formats supported by Pandoc.

  Example for reStructuredText (.rst):

  pandoc --from=gfm --to=rst https://raw.githubusercontent.com/pts/pdfsizeopt/master/README.md

  (You need to merge this PR for the above URL to work, of course 😇 To test with the modified README from this PR, use: pandoc --from=gfm --to=rst https://raw.githubusercontent.com/pts/pdfsizeopt/34a3347b1cc95fb273de5c82f3ec6616c478c2c7/README.md)

Besides just converting the README, I've also reduced some textual redundancy, added a table of contents and fixed some typos.

The result (rendered by GitHub) can be viewed here.

cf. #96

[pts]

Thank you for your work on this!

I've done the conversion myself, the result is in https://github.com/pts/pdfsizeopt/commit/137fa64f27de922b3beb270939aa31cf73fdd173