[chore] add README to checkapi (#38996)

Follow up to #38847 - adding a README for checkapi tool.

Author: atoulme

cmd/checkapi/README.md

@@ -0,0 +1,31 @@
+# CheckAPI
+
+**NOTE: CheckAPI is in development and immature. It is used primarily for opentelemetry-collector-contrib. Contributions are welcome.**
+
+CheckAPI is a go tool that parses the AST tree of a Go module, identifying Golang APIs such as structs and functions
+and enforcing rules against them.
+
+This is particularly useful to reduce the API surface of a Go module to a specific set of functions.
+
+## Running CheckAPI
+
+```shell
+$> checkapi -folder . -config config.yaml
+```
+
+## Configuration
+
+The configuration file is in yaml format:
+
+```yaml
+ignored_paths:
+  - <exact relative paths of Golang modules to ignore>
+allowed_functions:
+  - <at least one function match must be present.>
+  - name: <name of function>
+    parameters: <list of parameters by type>
+    return_types: <list of return types>
+
+ignored_functions:
+  - <regular expressions of ignored functions. At least one match must be present to ignore the function.>
+```