Merge README.md

Merge the 2 pages to only have one place with all the information (and clearer!)

Follow #395 ...

In fact, the main README.md file give not enough information and duplicate information given in the doc/README.md. But it prevent the user to look for more complete documentation in the doc/README.md file.

So I merged the 2 files and add some doc, make it clearer (I hope),.. and add some spelling faults! :(

Have a look before merging.... ;)