docs: add a second way to report security issues (jxmorris12#258)

Author: mdevolde

.github/pgp-key.asc

@@ -0,0 +1,15 @@
+-----BEGIN PGP PUBLIC KEY BLOCK-----
+Comment: 3591 093D FD82 D3C6 DC54  8ED4 1B36 9431 C0BE 4BF0
+Comment: Martin Devolder <martin.devolder2@gmail.com>
+
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+=CLKq
+-----END PGP PUBLIC KEY BLOCK-----

SECURITY.md

@@ -10,3 +10,9 @@ Only the latest stable release receives security fixes. There is only one suppor
 
 Use GitHub's private vulnerability reporting feature instead:
 [Report a vulnerability](https://github.com/jxmorris12/language_tool_python/security/advisories/new).
+
+Alternatively, you can email security issues to the maintainers at [martin.devolder2@gmail.com](mailto:martin.devolder2@gmail.com). Please encrypt sensitive information using the maintainer's PGP key:
+
+- **Fingerprint:** `3591 093D FD82 D3C6 DC54  8ED4 1B36 9431 C0BE 4BF0`
+- **Key file:** [publickey.asc](./.github/pgp-key.asc)
+- **Keyserver:** [keys.openpgp.org](https://keys.openpgp.org/search?q=martin.devolder2%40gmail.com)