emphasize the policy for actionlint's features at the top of the contribution guide

Author: rhysd

CONTRIBUTING.md

@@ -1,3 +1,15 @@
+# Policy for actionlint's features
+
+- actionlint focuses on detecting mistakes. Feature requests and patches for checks related to code style or
+  conventions are generally not accepted.
+- actionlint tries to keep [the configuration](docs/config.md) as minimal as possible. Feature requests and patches
+  for checks that require user configurations are generally not accepted.
+
+These are important to keep actionlint useful and convenient for everyone. I believe that no one wants to create and
+maintain a heavy configuration file just for linting CI workflows.
+
+It's helpful to check if a similar patch has been rejected in the past before submitting it.
+
 # Reporting an issue
 
 To report a bug, please submit a new ticket on GitHub. It's helpful to search similar tickets before making it.
@@ -11,8 +23,6 @@ investigate.
 # Sending a patch
 
 Thank you for taking your time to improve this project. To send a patch, please submit a new pull request on GitHub.
-It's helpful to check if a similar patch was rejected in the past before making it. actionlint focuses on detecting
-mistakes so adding rules related to styles or conventions is basically not accepted.
 
 https://github.com/rhysd/actionlint/pulls