Add SECURITY.md (#5536)

Adds security.md to be consistent with github's interface

CONTRIBUTORS.rst

@@ -392,6 +392,7 @@ Contributors
 * Pavel Denisov
 * Mikael Engström
 * Zac Connelly
+* Sarath Kumar Somana
 
 Translators
 ===========

SECURITY.md

@@ -0,0 +1,4 @@
+# Security
+We take the security of Wagtail, and related packages we maintain, seriously. If you have found a security issue with any of our projects please email us at security@wagtail.io so we can work together to find and patch the issue. We appreciate responsible disclosure with any security related issues, so please contact us first before creating a Github issue.
+
+If you want to send an encrypted email (optional), the public key ID for security@wagtail.io is 0x6ba1e1a86e0f8ce8, and this public key is available from most commonly-used keyservers.