doc: add readme

Signed-off-by: Florian Bauer <florian@fsrv.xyz>

Author: fsrv-xyz

README.md

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+# OCSPCRL
+
+OCSPCRL is a minimal implementation of both a OCSP and CRL server in Golang. It provides the following http endpoints:
+
+- `/ocsp` - OCSP responder
+- `/crl` - CRL responder
+
+All what you need is to provide a CRL file, the root certificate and cert/key with extendedKeyUsage `OCSPSigning` to allow the OCSP server to sign the OCSP responses.
+When using OCSP, the certificate is checked against the CRL for validity.
+
+Synchronization of the CAs CRL is out of scope of this project. You can use any mechanism to update the CRL file. Just notify the ocspcrl server process via `SIGHUP` signal to reload the CRL file.