Add requirement for UPnP

Describe UPnP or NAT firewall issues for IPv4 and link to a generic explainer.

Author: hoffmang9

README.md

@@ -1,5 +1,5 @@
 # chia-blockchain
-Python 3.7 is used for this project. Make sure your default python version is >=3.7 by typing python3.
+Python 3.7 is used for this project. Make sure your default python version is >=3.7 by typing python3. You will need to enable [UPnP](https://www.homenethowto.com/ports-and-nat/upnp-automatic-port-forward/) on your router or add a NAT (for IPv4 but not IPv6) and firewall rule to allow TCP port 8444 access to your peer. These tend to be router make/model specific.
 
 ### Install on Debian/Ubuntu