Document troubleshooting of 301 response code

[alexbudarov]

Description of the bug or enhancement

see this forum topic
https://www.cuba-platform.com/discuss/t/getting-an-http-301-error-on-attempting-to-get-rest-oauth/9945

Would be nice to add troubleshooting section.

If developer is getting an HTTP 301 and redirect to "/app/" - it is redirect by VAADIN.
It means that project doesn't have REST API endpoints at all.
Possible problems:

• add-on is not properly attached to project
• servlet mapping somehow missed from web.xml