Update codecov.yml

Update codecov.yml

Update the codecov.yml to make the following functional changes.

* Do not require the CI testing to pass before posting results.
* Set red-yellow-green coverage percent from 50%-100%
* Allow a 1% drop in coverage to still be considered a pass.
* Reduce the size of the comment posted to the issue.

Additionally, the top level README.markdown has been updated
to include the codecov.io badge and the project summary reworded.

Reviewed-by: Prakash Surya <prakash.surya@delphix.com>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #6669