Add README

README.md

+# Fachschaft Computerlinguistik Public Keys
+
+* Address: <fs-coli@cl.uni-heidelberg.de>
+  * S/MIME: [`fs-coli@cl.uni-heidelberg.de.smime.pub.pem`](fs-coli@cl.uni-heidelberg.de.smime.pub.pem)
+  * OpenPGP: [`fs-coli@cl.uni-heidelberg.de.pgp.pub.asc`](fs-coli@cl.uni-heidelberg.de.pgp.pub.asc)
+
+## Get key data
+
+To get information about the public keys (e.g. fingerprints), first, download the keys (or clone the repository). On a Linux system with all necessary tooling installed, you can then use one of the following commands:
+
+### S/MIME
+
+```bash
+openssl x509 -in fs-coli@cl.uni-heidelberg.de.smime.pub.pem -text -fingerprint -noout
+```
+
+An S/MIME certificate has both a serial number (arbitrarily assigned by the issuer) and one or more fingerprints (automatically generated by one or more cryptographic hashing algorithms). Read more about the differences here:
+
+* [What is the difference between serial number and thumbprint? - Information Security Stack Exchange](https://security.stackexchange.com/questions/35691/what-is-the-difference-between-serial-number-and-thumbprint)
+* [What is the actual value of a certificate fingerprint? - Information Security Stack Exchange](https://security.stackexchange.com/questions/35691/what-is-the-difference-between-serial-number-and-thumbprint)
+
+### OpenPGP
+
+```bash
+gpg --show-keys --with-fingerprint fs-coli@cl.uni-heidelberg.de.pgp.pub.asc 
+```
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