Improve contributing setup docs (#3447)

CONTRIBUTING.md

@@ -15,14 +15,16 @@ To hack on Pulumi, you'll need to get a development environment set up. You'll w
 - [pipenv](https://github.com/pypa/pipenv)
 - [Golangci-lint](https://github.com/golangci/golangci-lint)
 - [Yarn](https://yarnpkg.com/)
+- [Istanbul](https://github.com/gotwarlost/istanbul)
 
 ## Getting dependencies on macOS
 
-You can easily get all required dependencies with brew
+You can easily get all required dependencies with brew and npm
 
 ```bash
 brew install node pipenv python@3 typescript yarn go golangci/tap/golangci-lint
-brew cask install dotnet
+brew cask install dotnet dotnet-sdk
+npm install -g istanbul
 ```
 
 ## Make build system
@@ -31,6 +33,12 @@ We use `make` as our build system, so you'll want to install that as well, if yo
 
 For historical reasons (which we'd [like to address](https://github.com/pulumi/pulumi/issues/1515)) our build system requires that the folder `/opt/pulumi` exists and is writable by the current user. If you'd like, you can override this location by setting `PULUMI_ROOT` in your environment. The build is known to fail if this doesn't exist, so you'll need to create it first.
 
+```bash
+mkdir /opt/pulumi
+sudo chown <your_user_name>: /opt/pulumi
+export PATH=/opt/pulumi:/opt/pulumi/bin:$PATH
+```
+
 Across our projects, we try to use a regular set of make targets. The ones you'll care most about are:
 
 0. `make ensure`, which restores/installs any build dependencies