Overhaul Markdown documentation. Spring connector still needs rework.

link check

rewrite main README

link check

rewrite core doc

rephrase heading for clarity

weird GFM indentation in code blocks

info on adding ServiceInfos

update docs for Heroku and CF connectors

improve docs for local connector

tweak description of service connectors

edits as discussed in #54

reword Spring registration
Merge pull request #59 from chrylis/documentation-improvements-59

Overhaul Markdown documentation. Spring connector still needs rework.