docs: favour GitHub security form over mail

IMHO, the GitHub process nowadays provides a sleek workflow and is better than sending mails back-and-forth.

It's okay for me to leave that way open (especially if reporters really want to report high sensitive stuff and e.g. PGP-encrypt them), but I guess we should slightly suggest/favor the GitHub form for ease of maintenance.

@elrido what do you think?

SECURITY.md

@@ -9,13 +9,15 @@
 
 ## Reporting a Vulnerability
 
-You can send us email at security@privatebin.org. You should be able to get
+We suggest to [use the corresponding GitHub form](https://github.com/PrivateBin/PrivateBin/security/advisories/new)
+to report a new vulnerability directly on GitHub. [It can be handled there](https://docs.github.com/code-security/how-tos/report-and-fix-vulnerabilities/report-privately)
+and all necessary steps like verifying the vulnerability, crediting the finder
+and drafting a security advisory will be done there.
+
+You can also send us email at security@privatebin.org. You should be able to get
 a response within a week (usually during the next weekend). The respondee will
 reply from their personal address and can offer you their GPG public key to
 support end-to-end encrypted communication on sensitive topics or attachments.
 
-You can also [use the corresponding GitHub form](https://github.com/PrivateBin/PrivateBin/security/advisories/new)
-to report a new vulnerability directly on GitHub.
-
 You can also contact us via the regular issue tracker if the risk of early
 publication is low or you would request input from other PrivateBin users.