Update prometheus server compatibility for 0.8.x. (#249)

Author: qingling128

README.md

@@ -122,6 +122,7 @@ The matrix below lists the versions of Prometheus Server and other dependencies
 | **0.5.x**       | 2.6, 2.11                                 | 2.5                                       |
 | **0.6.x**       | 2.11                                      | 2.5                                       |
 | **0.7.x**       | 2.10, 2.11, 2.13, 2.15, 2.16, 2.18, 2.19  | 2.5                                       |
+| **0.8.x**       | 2.10, 2.11, 2.13, 2.15, 2.16, 2.18, 2.19  | 2.5                                       |
 
 ## Alternatives