Add documentation

[randycoulman]

Add full documentation, including:

• Fleshing out the README with more information about how to use the library and add it to a supervision tree
• Documentation for the ConfigCat website
• Adding module documentation for publishing to hexdocs.pm

[randycoulman]

From @laliconfigcat in #2 (comment):

We'd like to use the same format for the readme.md as the other SDKs.

Getting started
Sample/Demo apps
Polling Modes
Need help?
Contributing
About ConfigCat

[igorescobar]

@laliconfigcat I was looking at the Ruby SDK Readme file and it mentions also the public documentation (on the Configcat website). How do you suggest us to deal with this?

[igorescobar]

Talking with @laliconfigcat over Slack he mentioned that the website docs are here:
https://github.com/configcat/docs

So we can also just send a PR there and then reference it here on the README.

[igorescobar]

I'm starting to work on the README.md draft

[igorescobar]

Working on the " Documentation for the ConfigCat website"