OCSP request parsing (#4393)

* add public_bytes to OCSPRequest

* review feedback

* OCSP request parsing

* change some prose

* add __len__ as a required method
diff --git a/docs/x509/index.rst b/docs/x509/index.rst
index ec47fe6..ef51fbf 100644
--- a/docs/x509/index.rst
+++ b/docs/x509/index.rst
@@ -10,6 +10,7 @@
 
     tutorial
     certificate-transparency
+    ocsp
     reference
 
 .. _`public key infrastructure`: https://en.wikipedia.org/wiki/Public_key_infrastructure