Add step for disabling SIP to README.macOS.md
We've confirmed that these steps work on a brand new installation of macOS Sierra or a
brand new installation of macOS Sierra with [Pivotal's workstation-setup](https://github.com/pivotal/workstation-setup)
+## Step: Disable System Integrity Protection
+
+Note that you may need to disable System Integrity Protection in order to bring
+up the gpdemo cluster. Without doing this, psql commands run in child processes
+spawned by gpinitsystem may have the DYLD_* environment variables removed from
+their environments.
+
## Step: install needed dependencies. This will install homebrew if missing
```
./README.macOS.bash
