Add Quickstart section to the top level README

Requested internally by @trvrb to point to add a Quickstart section
to point people to the primary workflow of the repo.¹

¹ https://bedfordlab.slack.com/archives/C0KQCN813/p1705696505953639

README.md

@@ -7,6 +7,17 @@ This repository contains two workflows for the analysis of Zika virus data:
 
 Each folder contains a README.md with more information.
 
+## Quickstart
+
+Follow the [standard installation instructions](https://docs.nextstrain.org/page/install.html) for Nextstrain's suite of software tools.
+
+Then run the default phylogenetic workflow via:
+```
+cd phylogenetic/
+nextstrain build .
+nextstrain view .
+```
+
 ## Documentation
 
 - [Contributor documentation](./CONTRIBUTING.md)