README.adoc revision

Several changes to improve readme.adoc (started as a minnor typo-fixing). 
- changed ending punctuation. Added . where missing and : where deemed appropiate. Checked with other asciidoctor documentation like the maven-plugin or the reference.
- changed . by - in asciidoctor snapshot to be the same as its real format (that's maybe, beign to picky)
- changed order of titles in blocks according to asciidoc syntax reference. The current configuration works but the syntax reference and other docs use the opposite position. See, 'Code block with title and syntax highlighting' in http://asciidoctor.org/docs/asciidoc-syntax-quick-reference/#source-code
- some typos corrected
Actually, the prefix needed here is highly dependent on how you are publishing the content. The double forward slash will use whatever the requesting page uses...so long as you are viewing the page over an HTTP protocol. If you are viewing the page locally (i.e., `file://`), then you have to be explicit. That's when you usually choose `http://`, though I would recommend `https://` by default here.