repo: Add README

This change adds the beginning of a README, which covers basic topics
such as:

* how to build
* how to report vulnerabilities
* how to contribute

Tested: N/A

Bug: linear/CUS-360

README.md

@@ -0,0 +1,67 @@
+# engflow_auth
+
+This repository provides `engflow_auth`, a [Bazel credential helper](https://blog.engflow.com/2023/10/20/secure-builds-with-credential-helpers/) that helps you automatically obtain and securely store EngFlow authentication tokens.
+
+## Installation
+
+### One-time setup
+
+1. Download the appropriate binary from the latest [release
+   page](https://github.com/EngFlow/auth/releases/latest)
+1. Copy the downloaded binary to a directory on the system `$PATH` and mark as
+   executable (if necessary)
+1. Configure `.bazelrc`: In the `.bazelrc` file of either your project or user,
+   add a `build` flag that sets `--credential_helper` for your cluster. For
+   instance:
+
+   ```
+   build:engflow --credential_helper=example.cluster.engflow.com=/path/to/engflow_auth
+   ```
+
+   would configure the credential helper correctly when `--config=engflow` is
+   passed to a bazel invocation. See [Bazel's config
+   documentation](https://bazel.build/run/bazelrc) for more info on bazelrc
+   files, and [EngFlow setup
+   documentation](https://docs.engflow.com/re/client/bazel-first-time.html#4-set-up-bazelrc)
+   for EngFlow-specific setup and tips.
+
+### Use
+
+Each day, run `engflow_auth login [CLUSTER URL]` to obtain an auth credential;
+the application will emit a URL to visit to complete the login process.
+
+## Reporting Issues
+
+To report security vulnerabilities on `engflow_auth`, please send an email to
+[email protected] containing:
+
+* impact of the bug/vulnerability
+* steps to reproduce the issue
+* summary of expected vs. actual behavior observed
+
+For usability bugs and feature requests, please contact us through your DSE or
+via our [existing support
+channels](https://docs.engflow.com/support/get-day-to-day-support.howto.html).
+
+## Building
+
+The CLI can be built via either the Go toolchain or Bazel; released binaries are
+built via Bazel.
+
+To build release binaries, run:
+
+```
+bazel build --config=release //:release_artifacts
+```
+
+## Contributing
+
+We are not accepting pull requests from external contributors at this time due
+to both legal and technical reasons.
+
+The best way to report serious bugs/vulnerabilities is via email to
+[email protected] (see above section);
+
+If you are an EngFlow customer and would like feature additions or
+quality-of-life fixes, please discuss these with your DSE to get them
+appropriately prioritized.