Documentation: document connectors format.

Documentation/connectors-configuration.md

+# Configuring Connectors
+
+Connectors connect dex to authentication providers. dex needs to have at least one connector configured so that users can log in.
+
+## Configuration Format
+
+The dex connector configuration format is a JSON array of objects, each with an ID and type, in addition to whatever other configuration is required, like so:
+
+```
+[
+    {
+        "id": "local",
+        "type": "local"
+    }, {
+        "id": "Google",
+        "type": "oidc",
+        ...<<more config>>...
+    } 
+]
+```
+
+The additional configuration is dependent on the specific type of connector.
+
+### `local` connector
+
+The `local` connector allows email/password based authentication hosted by dex itself. It is special in several ways:
+
+* There can only be one `local` connector in your configuration.
+* The id must be `local`
+* No other configuration is required
+
+When the `local` connector is present, users can authenticate with the "Log in With Email" button on the authentication screen.
+
+The configuration for the local connector is always the same; it looks like this:
+
+```
+    {
+        "id": "local",
+        "type": "local"
+    }
+```
+
+### `oidc` connector
+
+This connector config lets users authenticate with other OIDC providers. In addition to `id` and `type`, the `oidc` connector takes the following additional fields:
+
+* issuerURL: a `string`. The base URL for the OIDC provider. Should be a URL with an `https` scheme.
+
+* clientID: a `string`. The OIDC client ID.
+
+* clientSecret: a `string`. The OIDC client secret.
+
+* trustedEmailProvider: a `boolean`. If true dex will trust the email address claims from this provider and not require that users verify their emails.
+
+In order to use the `oidc` connector you must register dex as an OIDC client; this mechanism is different from provider to provider. For Google, follow the instructions at their [developer site](https://developers.google.com/identity/protocols/OpenIDConnect?hl=en). Regardless of your provider, registering your client will also provide you with the client ID and secret.
+
+When registering dex as a client, you need to provide redirect URLs to the provider. dex requires just one:
+
+```
+https://$DEX_HOST:$DEX_PORT/auth/$CONNECTOR_ID/callback
+```
+
+`$DEX_HOST` and `$DEX_PORT` are the host and port of your dex installation. `$CONNECTOR_ID` is the `id` field of the connector for this OIDC provider.
+
+Here's what a `oidc` connector looks like configured for authenticating with Google; the clientID and clientSecret shown are not usable. We consider Google a trusted email provider because the email address that is present in claims is for a Google provisioned email account (eg. an `@gmail.com` address)
+
+```
+    {
+        "type": "oidc",
+        "id": "google",
+        "issuerURL": "https://accounts.google.com",
+        "clientID": "$DEX_GOOGLE_CLIENT_ID",
+        "clientSecret": "$DEX_GOOGLE_CLIENT_SECRET",
+        "trustedEmailProvider": true
+    }
+```
+
+## Setting the Configuration
+
+To set a connectors configuration in dex, put it in some temporary file, then use the dexctl command to upload it to dex:
+
+```
+dexctl -db-url=$DEX_DB_URL set-connector-configs /tmp/dex_connectors.json
+```