Add a SECURITY.md file (#6051)

.bumpversion.cfg

@@ -5,3 +5,7 @@ tag = True
 tag_name = {new_version}
 
 [bumpversion:file:scrapy/VERSION]
+
+[bumpversion:file:SECURITY.md]
+parse = (?P<major>\d+)\.(?P<minor>\d+)\.x
+serialize = {major}.{minor}.x

SECURITY.md

@@ -0,0 +1,12 @@
+# Security Policy
+
+## Supported Versions
+
+| Version | Supported          |
+| ------- | ------------------ |
+| 2.11.x     | :white_check_mark: |
+| < 2.11.x   | :x:                |
+
+## Reporting a Vulnerability
+
+Please report the vulnerability using https://github.com/scrapy/scrapy/security/advisories/new.