doc: crypto: document SPKAC additions

Document the SPKAC functionality that was added in commit 7bf46ba.

doc/api/crypto.markdown

@@ -474,6 +474,26 @@ such as in the generation of encryption keys.
 
 Usage is otherwise identical to `crypto.randomBytes`.
 
+## Class: Certificate
+
+The class used for working with signed public key & challenges. The most
+common usage for this series of functions is when dealing with the `<keygen>`
+element. http://www.openssl.org/docs/apps/spkac.html
+
+Returned by `crypto.Certificate`.
+
+### Certificate.verifySpkac(spkac)
+
+Returns true of false based on the validity of the SPKAC.
+
+### Certificate.exportChallenge(spkac)
+
+Exports the encoded public key from the supplied SPKAC.
+
+### Certificate.exportPublicKey(spkac)
+
+Exports the encoded challenge associated with the SPKAC.
+
 ## crypto.DEFAULT_ENCODING
 
 The default encoding to use for functions that can take either strings