Note that sha3 (keccak) is supported

I noticed that sha3 is supported as of DaGenix/rust-crypto#346, but not yet mentioned in the readme.

Author: cosmycoder

README.md

@@ -69,5 +69,6 @@ it will support even more in the future. Currently supported algorithms include:
 * Scrypt
 * Sha1
 * Sha2 (All fixed output size variants)
+* Sha3
 * Sosemanuk
 * Whirlpool