Adds warning about crawling with basic auth (#669)

Closes webrecorder/browsertrix#1950 over here
too

### Changes
- Adds a warning about using basic auth
- Adds a link to MDN because learning and cross referencing is fun!

docs/docs/user-guide/yaml-config.md

@@ -49,7 +49,10 @@ seeds:
 
 ## HTTP Auth
 
-Browsertrix Crawler supports HTTP Basic Auth, which can be provide on a per-seed basis as part of the URL, for example:
+!!! warning "HTTP basic auth credentials are written to the archive"
+    We recommend exercising caution and only archiving with dedicated archival accounts, changing your password or deleting the account when finished.
+
+Browsertrix Crawler supports [HTTP Basic Auth](https://developer.mozilla.org/en-US/docs/Web/HTTP/Authentication), which can be provide on a per-seed basis as part of the URL, for example:
 `--url https://username:[email protected]/`.
 
 Alternatively, credentials can be added to the `auth` field for each seed: