Update sentry docs for source map retrieval

docs/sentry.md

@@ -4,6 +4,8 @@ The Starter App supports error logging with [Sentry](https://sentry.io/) out of
 that required environment variables are set in place. Other logging solutions can also be used but
 the Sentry client comes already strapped into the Starter App.
 
+## Setting up Sentry keys
+
 To enable the Sentry error logging a DSN, _Data Source Name_ has to be provided as an environment
 variable. Browser and Node environments both require their own keys. The key names are as follows:
 
@@ -19,3 +21,19 @@ If the Sentry DSN keys are not provided Starter App will log errors to the conso
 Sentry setup is implemented in [util/log.js](../src/util/log.js) and
 [server/log.js](../server/log.js) so refer to those files to change the external logging service or
 the logging logic in general.
+
+## Source map retrieval
+
+By default Sentry fetches the source maps for minified javascript files. However, this might now work
+in case some sort of authentication, e.g. basic auth, is required to reach your application. To
+circumvent this a security token can be configured to Sentry. Sentry then adds this token to the
+source maps request header so that the server hosting Starter App can then be configured to let those
+requests access the source maps.
+
+For basic auth the security token can be configured as follows:
+
+In *Project > Settings > General > Client Security* set the following values
+
+* **Security token** - `Basic <your username:password in Base 64>`
+* **Security token header** - `Authorization`
+