add a README

Author: marten-seemann

README.md

@@ -0,0 +1,31 @@
+# GitHub Action: multiple-go-modules
+
+Go allows a Git repository to contain multiple Go modules (i.e. multiple directory trees that have their own `go.mod` file).
+When running Go tools like `go test ./...` or `go vet ./...` from the parent directory, it will only recurse into directories belonging to the same Go module.
+In order to run the command in all subdirectories, no matter which module they belong to, the command has to be invoked separately for every Go module.
+
+This GitHub Action makes it easy to do this.
+
+## Usage
+
+Take the following example, running a command (or multiple commands) directly:
+
+```yml
+steps:
+  - name: Code Quality Checks
+    run: |
+      go vet ./...
+      gofmt .
+```
+
+Using this action, this would run the same command(s) in all Go modules:
+
+```yml
+steps:
+  - name: Code Quality Checks
+    uses: protocol/multiple-go-modules@master
+    with:
+      run: |
+        go vet ./...
+        gofmt .
+```