Merge pull request #158 from miri64/security.txt-guide

security.txt: link to policy via the more permanent guide page

Author: bergzand

.well-known/security.txt

@@ -1,5 +1,5 @@
 Contact: mailto:security@riot-os.org
 Expires: 2028-01-13T10:09:55.000Z
-Policy: https://git.riot-os.org/RIOT-OS/RIOT/src/branch/master/SECURITY.md
+Policy: https://guide.riot-os.org/general/security/
 Preferred-Languages: en
 Encryption: https://riot-os.org/assets/keys/security.asc