Added warnings for previous gitbook users (#7945)

Close #7939

Co-authored-by: Santos Gallegos <[email protected]>

Author: FatGrizzly

docs/custom_domains.rst

@@ -127,6 +127,11 @@ You will need to update your CAA records to allow us to issue the certificate.
 .. _Cloudflare CAA FAQ: https://support.cloudflare.com/hc/en-us/articles/115000310832-Certification-Authority-Authorization-CAA-FAQ
 .. _Amazon CAA guide: https://docs.aws.amazon.com/acm/latest/userguide/setup-caa.html
 
+.. note::
+ 
+   If your custom domain was previously used in GitBook, contact GitBook support (via live chat in their website)
+   to remove the domain name from their DNS Zone in order for your domain name to work with Read the Docs,
+   else it will always redirect to GitBook.
 
 Proxy SSL
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