Add API compatibility note

docs/developer-guide/api_compatibility.md

@@ -0,0 +1,25 @@
+# API Compatibility
+
+This is an internal documentation summarizing the currently supported SDK and TFLint versions and any compatibility caveats.
+
+Protocol version: 11  
+SDK version: v0.12.0+  
+TFLint version: v0.40.0+  
+
+- `Only` option is only supported by SDK v0.13.0+.
+  - https://github.com/terraform-linters/tflint/pull/1516
+  - https://github.com/terraform-linters/tflint-plugin-sdk/pull/198
+- Schema mode is only supported by TFLint v0.42.0+. v0.41 ignores this mode.
+  - https://github.com/terraform-linters/tflint/pull/1530
+  - https://github.com/terraform-linters/tflint-plugin-sdk/pull/201
+- `VersionConstraint` is only supported by TFLint v0.42.0+. v0.41 ignores this constraint.
+  - https://github.com/terraform-linters/tflint/pull/1535
+  - https://github.com/terraform-linters/tflint-plugin-sdk/pull/202
+- `SDKVersion` is only supported by SDK v0.13.0+. v0.12 does not return a version.
+  - https://github.com/terraform-linters/tflint-plugin-sdk/pull/203
+- `each.*` and `count.*` are only supported by SDK v0.13.0+ and TFLint v0.42.0+.
+  - https://github.com/terraform-linters/tflint/pull/1537
+  - https://github.com/terraform-linters/tflint-plugin-sdk/pull/205
+- Expand mode is only supported by TFLint v0.42.0+.
+  - https://github.com/terraform-linters/tflint/pull/1537
+  - https://github.com/terraform-linters/tflint-plugin-sdk/pull/208