Indicate level of support for RHEL/CentOS

Despite the test builds for CentOS, it's probably safe to mark them as
unknown until they've been used a bit more heavily.

README.md

@@ -17,6 +17,10 @@ MongoDB support matrix:
 | Ubuntu 16.04 | :no_entry: | :x: | :x: | :x:|
 | Debian 7.x | :no_entry: | :interrobang: | :interrobang: | :interrobang:|
 | Debian 8.x | :no_entry: | :x: | :x: | :x:|
+| CentOS 6.x | :no_entry: | :interrobang: | :interrobang: | :interrobang: |
+| CentOS 7.x | :no_entry: | :interrobang: | :interrobang: | :interrobang: |
+| RHEL 6.x | :no_entry: | :interrobang: | :interrobang: | :interrobang: |
+| RHEL 7.x | :no_entry: | :interrobang: | :interrobang: | :interrobang: |
 
 :white_check_mark: - fully tested, should work fine  
 :interrobang: - will be added testing suite soon  

meta/main.yml

@@ -16,6 +16,10 @@ galaxy_info:
   - name: Debian
     versions:
     - wheezy
+  - name: EL
+    versions:
+      - 6
+      - 7
   galaxy_tags:
   - database
   - database:nosql