Improve the README

Signed-off-by: Matt Moore <mattmoor@chainguard.dev>

README.md

@@ -1,2 +1,75 @@
-# octo-sts
-A Github App that acts like a Security Token Service (STS) for the Github API
+# `octo-sts`: an STS for Github
+
+This repository holds a Github App called `octo-sts` that acts like a Security
+Token Service (STS) for the Github API. Using this App, workloads running
+essentially anywhere that can produce OIDC tokens can federate with this App's
+STS API in order to produce short-lived tokens for interacting with Github.
+
+**_The ultimate goal of this App is to wholly eliminate the need for Github
+Personal Access Tokens (aka PATs)._**
+
+> _[The full design document.](inky.wtf/github-sts)_
+
+## Setting up workload trust
+
+For the App to produce credentials that work with resources in your organization
+it must be installed into the organization and have access to any repositories
+that you will want workloads to be able to interact with.  Unfortunately due to
+limitations with Github Apps, the App must ask for a superset of the permissions
+needed for federation, so the full set of permissions the App requests will be
+large, but with one exception (`contents: read` reading policy files) the App
+only creates tokens with these scopes based on the "trust policies" you have
+configured.
+
+### The Trust Policy
+
+Trust policies are checked into `.github/chainguard/{name}.yaml`, and consist of
+a few key parts:
+1. The claim matching criteria for federation,
+2. The permissions to grant the identity, and
+3. (for Org-level policies) The list of repositories to grant access.
+
+Here is a simple example that allows the Github actions workflows in
+`chainguard-dev/foo` running on the `main` branch to read the repo contents and
+interact with issues:
+
+```yaml
+issuer: https://token.actions.githubusercontent.com
+subject: repo:chainguard-dev/foo:ref:refs/heads/main
+
+permissions:
+  contents: read
+  issues: write
+```
+
+The Trust Policy can also match the issuer, subject, and even custom claims with
+regular expressions.  For example:
+
+```yaml
+issuer: https://accounts.google.com
+subject_pattern: '[0-9]+'
+claim_pattern:
+  email: '.*@chainguard.dev'
+
+permissions:
+  contents: read
+```
+
+This policy will allow OIDC tokens from Google accounts of folks with a
+Chainguard email address to federate and read the repo contents.
+
+### Federating a token
+
+The Github App implements the Chainguard `SecurityTokenService` GRPC service
+definition [here](https://github.com/chainguard-dev/sdk/blob/main/proto/platform/oidc/v1/oidc.platform.proto#L13-L28).
+
+If a `${TOKEN}` suitable for federation is sent like so:
+```
+curl -H "Authorization: Bearer ${TOKEN}" \
+  "https://octo-sts-j2wqachcbq-uc.a.run.app/sts/exchange?scope=${REPO}&identity=${NAME}"
+```
+
+The App will attempt to load the trust policy from
+`.github/chainguard/${NAME}.yaml` from `${REPO}` and if the provided `${TOKEN}`
+satisfies those rules, it will return a token with the permissions in the trust
+policy.