Update description on compatibility section. (#153)

Author: StevenYCChou

README.md

@@ -111,14 +111,14 @@ When using Counter Aggregator you would usually want to configure a restrictive
 
 ## Compatibility
 
-The matrix below lists the versions of Prometheus Server and other dependencies that have been qualified to work with releases of `stackdriver-prometheus-sidecar`.
-
-| sidecar version | **Prometheus 2.4.3**| **Prometheus 2.5.x**| **Prometheus 2.6.x**|
-|-----------------|---------------------|---------------------|---------------------|
-| **0.2.x**       |          ✓          |          ✗          |          ✗          |
-| **0.3.x**       |          ✓          |          ✗          |          ✓          |
-| **0.4.x**       |          ?          |          ✗          |          ✓          |
-| **0.5.x**       |          ?          |          ✗          |          ✓          |
+The matrix below lists the versions of Prometheus Server and other dependencies that have been qualified to work with releases of `stackdriver-prometheus-sidecar`. If the matrix does not list whether they are compatible, please assume they are not verified yet. Feel free to contribute to the matrix if you have run the end-to-end test between a version of `stackdriver-prometheus-sidecar` and Prometheus server. 
+
+| sidecar version | **Prometheus 2.4.3**| **Prometheus 2.5.x**| **Prometheus 2.6.x**|**Prometheus 2.7+  **|
+|-----------------|---------------------|---------------------|---------------------|---------------------|
+| **0.2.x**       |          ✓          |          ✗          |          ✗          |          ?          |
+| **0.3.x**       |          ✓          |          ✗          |          ✓          |          ?          |
+| **0.4.x**       |          ?          |          ✗          |          ✓          |          ?          |
+| **0.5.x**       |          ?          |          ✗          |          ✓          |          ?          |
 
 - ✓: Verified as compatible.
 - ✗: Verified as incompatible.