Rejiggered README

Incorporating all of what was on wiki main page. Serves as a table of contents linking to other docs (narrative doc/*.md and/or generated yarddocs). Examples moved to docs relevant to those examples.

* Where appropriate docs exist, I link directly to md narrative docs in source `./doc`-- I use relative links, which will work on github, and will also work in yard generated docs (so long as you tell yard to include all files in ./doc!)

* Where no such narrative docs exist, I link to class yardocs at http://ruby-concurrency.github.io/concurrent-ruby/ -- I need to use full absolute URLs for those. 

Do you plan to leave the generated yarddocs at http://ruby-concurrency.github.io/concurrent-ruby/?  Another alternative would be using rdoc.info, although you do have more control generating your own in github pages. 

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**Next:** Assuming you are happy with the current situation of generating yarddoc to github pages, if you can give me the command line (or other means?) you have been using to generate yarddocs in the past, I'll work on making sure that's all working right and possibly make it more convenient:

* Make sure yarddocs include narrative docs at `./doc/*.md`
* Maybe make a rake task to automatically generate yarddocs and upload them to github pages? Make it easier to keep docs updated with every release (maybe even automatically make the gem release task generate and upload docs?)
* Look into if there's a way to have generated yarddocs automatically include links to source on github, I always find it more convenient when it does that.
* Add links on relevant narrative `docs/*.md`, to generated yarddoc on http://ruby-concurrency.github.io/concurrent-ruby/, for classes discussed.