Add autotools to release languages.

.github/mergeable.yml

@@ -11,8 +11,8 @@ mergeable:
                 regex: 'release notes: yes'
                 message: 'Please include release notes: yes'
             - must_include:
-                regex: '^(c#|c\+\+|cleanup|conformance tests|integration|java|javascript|go|objective-c|php|python|ruby|bazel|cmake|protoc)'
-                message: 'Please include at least a language label (e.g., c++, java, python). Or apply one of the following labels: bazel, cmake, cleanup, conformance tests, integration, protoc.'
+                regex: '^(autotools|bazel|c#|c\+\+|cleanup|cmake|conformance tests|integration|go|java|javascript|objective-c|php|protoc|python|ruby)'
+                message: 'Please include at least a language label (e.g., c++, java, python). Or apply one of the following labels: autotools, bazel, cmake, cleanup, conformance tests, integration, protoc.'
           - must_include:
               regex: 'release notes: no'
               message: 'Please include release notes: no'