Add notes on getting gitian builds going

contrib/gitian-descriptors/README

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+Gavin's notes on getting gitian builds up and running:
+
+You need the right hardware: you need a 64-bit-capable CPU with hardware virtualization support (Intel VT-x or AMD-V). Not all modern CPUs support hardware virtualization.
+
+You probably need to enable hardware virtualization in your machine's BIOS.
+
+You need to be running a recent version of 64-bit-Ubuntu, and you need to install several prerequisites:
+  sudo apt-get install apache2 git apt-cacher-ng python-vm-builder qemu-kvm
+
+Sanity checks:
+  sudo service apt-cacher-ng status   # Should return apt-cacher-ng is running
+  ls -l /dev/kvm   # Should show a /dev/kvm device
+
+Once you've got the right hardware and software:
+
+    git clone git://github.com/bitcoin/bitcoin.git
+    git clone git://github.com/devrandom/gitian-builder.git
+    mkdir gitian-builder/inputs
+    wget 'http://miniupnp.tuxfamily.org/files/download.php?file=miniupnpc-1.6.tar.gz' -O gitian-builder/inputs/miniupnpc-1.6.tar.gz
+
+    cd gitian-builder
+    bin/make-base-vm --arch i386
+    bin/make-base-vm --arch amd64
+    cd ..
+
+    # To build
+    cd bitcoin
+    git pull
+    cd ../gitian-builder
+    git pull
+    ./bin/gbuild --commit bitcoin=HEAD ../bitcoin/contrib/gitian.yml