docs: document fdp login/logout and auto-login

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

Author: sammuli

README.md

@@ -1 +1,10 @@
 # fdp
+
+## Authentication
+
+`fdp login` mints a SciToken via the pelican GitHub-OAuth flow and caches it
+under `~/.fdp/cache/<device>.token`. `fdp run ...` auto-acquires a token when
+none is valid and the session is interactive; set `FDP_NO_AUTO_LOGIN=1` to
+disable that (e.g. in batch jobs). `fdp logout` deletes the cached token.
+`fdp env` never launches the flow. Resolution order: `-t/--bearer-token`,
+then `$BEARER_TOKEN`, then the managed cache, then the legacy `~/.fdp/token`.