Create an easy to use gitian building script

I've written a script that automates the setup and building of binaries with gitian. All of the commands are pulled from various documentation on gitian building.

contrib/README.md

@@ -45,6 +45,9 @@ Scripts and notes for Mac builds.
 ### [RPM](/contrib/rpm) ###
 RPM spec file for building bitcoin-core on RPM based distributions
 
+### [Gitian-build](/contrib/gitian-build.sh) ###
+Script for running full gitian builds.
+
 Test and Verify Tools 
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