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

.github/codecov.yml

@@ -1,9 +1,30 @@
 codecov:
-  strict_yaml_branch: master # only use the latest copy on master branch
-
-comment: off
+  notify:
+    require_ci_to_pass: no
 
 coverage:
+  precision: 2
+  round: down
+  range: "50...100"
+
   status:
-    project: off
-    patch: off
+    project:
+      default:
+        threshold: 1%
+
+    patch:
+      default:
+        threshold: 1%
+
+parsers:
+  gcov:
+    branch_detection:
+      conditional: yes
+      loop: yes
+      method: no
+      macro: no
+
+comment:
+  layout: "header, sunburst, diff"
+  behavior: default
+  require_changes: no