add readme

Author: robherley

README.md

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+# go-test-action
+
+GitHub Action for running `go test ...` and getting rich summary and annotations as output.
+
+Powered by [Job Summaries](https://github.blog/2022-05-09-supercharging-github-actions-with-job-summaries/), this Action will generate a convenient interactive viewer for tests based on Go's [test2json](https://pkg.go.dev/cmd/test2json) output.
+
+## Example
+
+Tests are organized per package, with a brief summary of individual test results:
+
+![summary overview](docs/img/overview.png)
+
+Expand for per-test (with subtest) results and to view raw test output:
+
+![summary expanded](docs/img/expanded.png)
+
+## Inputs
+
+- `moduleDirectory` (optional): relative path to the directory containing the `go.mod` of the module you wish to test
+  - Default: `.`
+- `testArguments` (optional): arguments to pass to `go test`, `-json` will be prepended
+  - Default: `./...`
+