Add API auth docs

Author: rsaksida

docs/api_authentication.md

@@ -0,0 +1,40 @@
+# Google API Authentication
+
+Metadocs is compatible with the [Credentials](https://www.rubydoc.info/github/google/google-auth-library-ruby/Google/Auth/Credentials)
+object used by the official [googleauth](https://github.com/googleapis/google-auth-library-ruby)
+library.
+
+There are different ways to authenticate with Google:
+
+- on behalf of end users with OAuth2, for web or console applications;
+- with a service account.
+
+See the [googleauth](https://github.com/googleapis/google-auth-library-ruby) docs for more info.
+
+Either option will work with this library - what's important is to pass on the `Credentials` created
+by `googleauth`.
+
+The scopes you need to authorize for are provided in `Metadocs::GoogleDocument::RequiredScopes`.
+Currently only `https://www.googleapis.com/auth/documents` is needed.
+
+## Example
+
+```ruby
+credentials = Google::Auth::ServiceAccountCredentials.make_creds(
+  json_key_io: File.open('/path/to/service_account_json_key.json'),
+  scope: Metadocs::GoogleDocument::RequiredScopes
+)
+
+metadoc = Metadocs::Parser.parse(credentials, 'https://docs.google.com/document/d/...')
+```
+
+## Permissions
+
+Note that the credentials object must have permission to view the document you're trying to access.
+
+For on behalf authorization, that means the user you're accessing data on behalf of has permission
+to view the document.
+
+For service accounts, you can share the document with the service account
+e-mail that shows up in the `client_email` property of the service account credentials JSON file
+exported by Google.