docs: add readme

Author: echozio

README.md

@@ -0,0 +1,16 @@
+# commit-status-poster
+Posts a GitHub Commit Status using a GitHub App to a given reference
+
+## Running
+Requires the following environment variables:
+* `GITHUB_APP_ID`
+* `GITHUB_APP_INSTALLATION_ID`
+* `GITHUB_APP_PRIVATE_KEY_FILE`: Path pointing to a file containing the private key.
+* `GITHUB_COMMIT_STATUS_STATE`: One of `error`, `failure`, `pending`, `success`.
+* `GITHUB_COMMIT_STATUS_TARGET_URL`: URL for the "Details" link appearing next to the status.
+* `GITHUB_COMMIT_STATUS_DESCRIPTION`: Description describing the current status, e.g. "Deploying..."
+* `GITHUB_COMMIT_STATUS_CONTEXT`: The name that appears on GitHub. Must be the same when updating an
+	existing status.
+* `GITHUB_OWNER`
+* `GITHUB_REPO`
+* `GITHUB_REF`: The commit hash the status applies to.