Document required Node version

gatsby develop errors with any Node version earlier than 10.13, so I’ve added this
requirement to the package.json and set an .npmrc file to require it when `npm install`
is run.

My next step would be to define an `.nvmrc` to enable auto-switching to 10.13 but that’s
assuming:
- people use nvm
- we’re okay tracking the minimum node version in two places: `package.json` and `.nvmrc`

Let me know and I’ll update this PR.

.npmrc

@@ -0,0 +1 @@
+engine-strict=true

package.json

@@ -1,6 +1,9 @@
 {
   "name": "covid-tracking-project",
   "version": "0.0.1",
+  "engines": {
+    "node": ">=10.13"
+  },
   "description": "Collects information from 50 US states, the District of Columbia, and 5 other US territories to provide the most comprehensive testing data we can collect for the novel coronavirus, SARS-CoV-2.",
   "author": "The COVID Tracking Project",
   "homepage": "https://covidtracking.com",