Update CONTRIBUTING and explain the dependent repos

Author: hoffmang9

CONTRIBUTING.md

@@ -4,7 +4,7 @@ We are happy that you are taking a look at the code for Chia, a proof of space a
 
 A lot of fascinating new cryptography and blockchain concepts are used and implemented here.
 This repo includes the code for the Chia full node, farmer, and timelord (in src), which are all written in python.
-It also includes a verifiable delay function implementation under lib/chiavdf (in c/c++), and a proof of space implementation under lib/chiapos.
+It also includes a verifiable delay function implementation that it imports from the [chiavdf repo](https://github.com/Chia-Network/chiavdf) (in c/c++), and a proof of space implementation that it imports from the [chiapos repo](https://github.com/Chia-Network/chiapos). BLS signatures are imported from the [bls-signatures repo](https://github.com/Chia-Network/bls-signatures) as blspy. There is an additional dependency on the [chiabip158 repo](https://github.com/Chia-Network/chiabip158). For major platforms, binary and source wheels are shipped to PyPI from each dependent repo and then chia-blockchain can pip install those from PyPI or they can be prepackaged as is done for the Windows installer. On unsupported ploatforms, pip will fall back to the source distributions to be compiled locally.
 
 If you want to learn more about this project, read the [wiki](https://github.com/Chia-Network/chia-blockchain/wiki), or check out the [green paper](https://www.chia.net/assets/ChiaGreenPaper.pdf).