Add prerequisites

Author: nicolasvasilache

BUILD.md

@@ -6,6 +6,13 @@ For more information about setting up the config that we use to build the conda
 Our main goal with this decision is to make the build procedure extremely simple, both reproducible internally and extensible to new targets in the future.
 In particular the gcc-4 / gcc-5 ABI switch is not something we want to concern ourselves with at this point, we go for gcc-5.4.0.
 
+# Prerequisites
+Building TC from source requires `gmp`. To install on Ubuntu16.04:
+
+```sudo apt-get install libgmp3-dev`
+
+More generally, the [Dockerfile](https://github.com/facebookresearch/TensorComprehensions/blob/master/conda_recipes/docker-images/tc-cuda9.0-cudnn7.1-ubuntu16.04-devel/Dockerfile) shows the environment we are using for testing and building conda pacakges.
+
 # Conda from scratch (first time configuration)
 Choose and set an INSTALLATION_PATH then run the following: