docs: add security email info to readme

We have an email alias set up for team leads, so that external parties
have a single entrypoint to disclose security-related issues.
Once we settle on language here, we can duplicate this notice in other
Penumbra-related repositories.

README.md

@@ -41,7 +41,13 @@ For a high-level view of current work-in-progress and future items, check out ou
 [protobuf]: https://buf.build/penumbra-zone/penumbra
 [tm-install]: https://github.com/tendermint/tendermint/blob/master/docs/introduction/install.md#from-source
 
-# License
+
+## Security
+If you believe you've found a security-related issue with Penumbra,
+please disclose responsibly by contacting the Penumbra Labs team at
+[email protected].
+
+## License
 
 By contributing to penumbra you agree that your contributions will be licensed
 under the terms of both the [LICENSE-Apache-2.0](LICENSE-Apache-2.0) and the
@@ -50,4 +56,3 @@ under the terms of both the [LICENSE-Apache-2.0](LICENSE-Apache-2.0) and the
 If you're using penumbra you are free to choose one of the provided licenses:
 
 `SPDX-License-Identifier: MIT OR Apache-2.0`
-