readme: add security contact

readme.md

···
 
 Read the introduction to Tangled [here](https://blog.tangled.sh/intro).
 
-Documentation:
+## docs
+
 * [knot hosting
 guide](https://tangled.sh/@tangled.sh/core/blob/master/docs/knot-hosting.md)
 * [contributing
 guide](https://tangled.sh/@tangled.sh/core/blob/master/docs/contributing.md)—**read this before opening a PR!**
+
+## security
+
+If you've identified a security issue in Tangled, please email
+[[e-mail]](mailto:[e-mail]) with details!