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

README.markdown

@@ -1,9 +1,9 @@
-<p align="center"><img src="http://zfsonlinux.org/images/zfs-linux.png"/></p>
-ZFS is an advanced file system and volume manager which was originally
-developed for Solaris and is now maintained by the Illumos community.
+![img](http://zfsonlinux.org/images/zfs-linux.png)
 
-ZFS on Linux, which is also known as ZoL, is currently feature complete.  It
-includes fully functional and stable SPA, DMU, ZVOL, and ZPL layers. And it's native!
+ZFS on Linux is an advanced file system and volume manager which was originally
+developed for Solaris and is now maintained by the OpenZFS community.
+
+[![codecov](https://codecov.io/gh/zfsonlinux/zfs/branch/master/graph/badge.svg)](https://codecov.io/gh/zfsonlinux/zfs)
 
 # Official Resources
   * [Site](http://zfsonlinux.org)
@@ -16,4 +16,4 @@ Full documentation for installing ZoL on your favorite Linux distribution can
 be found at [our site](http://zfsonlinux.org/).
 
 # Contribute & Develop
-We have a separate document with [contribution guidelines](./.github/CONTRIBUTING.md).
+We have a separate document with [contribution guidelines](./.github/CONTRIBUTING.md).