Add security key into readme (#5223)

README.md

@@ -78,6 +78,8 @@ Wagtail is sponsored by [Torchbox](https://torchbox.com/). If you need help impl
 
 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](mailto: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.
+
 ### Contributing
 
 If you're a Python or Django developer, fork the repo and get stuck in! We run a separate group for developers of Wagtail itself at https://groups.google.com/forum/#!forum/wagtail-developers (please note that this is not for support requests).