Add contributing guidelines

#240 (comment)

Author: bendrucker

.github/CONTRIBUTING.md

@@ -0,0 +1,19 @@
+# Contributing
+
+## Accepted Rules
+
+Terraform Language rules implement recommendations from the [Terraform Language documentation](https://www.terraform.io/language). This ruleset _does not_ provide configurable rules for personal/team style or usage preferences. If you'd like to enforce stylistic rules beyond the official Terraform Language recommendations, you should [author your own ruleset plugin](https://github.com/terraform-linters/tflint/blob/master/docs/developer-guide/plugins.md).
+
+In rare circumstances, we may also accept rules that detect language usage errors that are _not_ already detected by `terraform validate`. 
+
+If you are unsure whether your proposed rule meets these criteria, [open a discussion](https://github.com/terraform-linters/tflint-ruleset-terraform/discussions/new?category=ideas) thread first before authoring a pull request.
+
+## Authoring a Rule
+
+Each rule should have:
+
+* A source file implementing the rule
+* Tests that check expected issues against different Terraform configurations to cover applicable cases
+* Documentation explaining the rule, its motivation, and how users should fix their configuration
+
+You will also need to add your rule to applicable [presets](https://github.com/terraform-linters/tflint-ruleset-terraform/blob/main/rules/preset.go).