Add building instructions (#5)

doc/BUILDING.md

@@ -0,0 +1,39 @@
+## Building nuTensor
+
+### Requirements
+
+Tools:
+* bash
+* python 3
+
+You will need both this nuTensor and the nuAssets repositories. These should both be placed in the same directory:
+```
+git clone https://github.com/geekprojects/nuAssets.git
+git clone https://github.com/geekprojects/nuTensor.git
+cd nuTensor
+```
+
+### Packaging
+You can now run the scripts that package everything up.
+These are bash scripts. They have only been tested on Linux and MacOS.
+
+#### For Firefox
+```
+tools/make-firefox.sh all
+```
+
+#### For Chrome/Chromium (Not yet tested)
+```
+tools/make-chromium.sh all
+```
+
+#### For Opera (Not yet tested)
+```
+tools/make-opera.sh
+```
+
+The installation package should now be found in dist/build/
+
+### Installing
+
+Follow the instructions in [README.md](README.md) to install it.