Add gem signing instructions

README.md

@@ -13,6 +13,16 @@ US Dollars.
 ## Installation
     gem install mtgox
 
+To ensure the code you're installing hasn't been tampered with, it's
+recommended that you verify the signature. To do this, you need to add my
+public key as a trusted certificate (you only need to do this once):
+
+    gem cert --add <(curl -Ls https://gist.github.com/sferik/4701180/raw/public_cert.pem)
+
+Then, install the gem with the high security trust policy:
+
+    gem install mtgox -P HighSecurity
+
 ## Alias
 After installing the gem, you can get the current price for 1 BTC in USD by
 typing `btc` in your bash shell simply by setting the following alias: