Added installation instructions for FastBoot users

[ijlee2]

Description

This addon uses nullish coalescing operator ??, introduced in ECMA2020 (in June 2020) and natively supported in Node starting v14.0. Because Ember's blueprint for config/targets.js uses last 1 versions for browsers, there's a good chance that ?? doesn't get transpiled anymore.

As a result, developers who use ember-cli-fastboot and ember-container-query can encounter this build error:

/var/folders/2z/93zyyhx13rs879qr8rzyxrb40000gn/T/broccoli-689520dxo26a682Mz/out-529-broccoli_merge_trees/assets/vendor.js:121232
  return this.args.features ?? {};
                             ^

SyntaxError: Unexpected token '?'
  at new Script (vm.js:88:7)
  at VMSandbox.eval (/my-app/node_modules/fastboot/src/vm-sandbox.js:13:22)
  at /my-app/node_modules/fastboot/src/ember-app.js:190:15
  at Array.forEach (<anonymous>)
  at EmberApp.loadAppFiles (/my-app/node_modules/fastboot/src/ember-app.js:187:21)
  at EmberApp.retrieveSandboxedApp (/my-app/node_modules/fastboot/src/ember-app.js:235:10)
  at new EmberApp (/my-app/node_modules/fastboot/src/ember-app.js:61:21)
  at FastBoot._buildEmberApp (/my-app/node_modules/fastboot/src/index.js:114:17)
  at new FastBoot (/my-app/node_modules/fastboot/src/index.js:52:10)
  at /my-app/node_modules/ember-cli-fastboot/index.js:330:29

At a glance, it's not obvious what developers need to do to fix this error. (It certainly wasn't to me when I had encountered this error.) Based on Jeff's feedback in #49, I decided to explicitly mention what to do in the README.

References

• SyntaxError: Unexpected token occurs with nullish collascing operator ember-fastboot/ember-cli-fastboot#764
• Add 'node: current' to the targets file on install ember-fastboot/ember-cli-fastboot#770