message: Document build/test process better
  1. `yarn test` runs `node ./out/test/runTest.js` which doesn't work because `out/test/runTest.js` does not exist yet.  Add `npx gulp prepare-test` step.
  2. Mention how to build `.vsix` extension.
diff: @@ -47,11 +47,15 @@ When submitting a PR, please fill out the template that is presented by GitHub w
  # Alternatively, build and run tests through gulp and npm scripts
  npx gulp build # build
  + npx gulp prepare-test # build tests
  yarn test # test (must close all instances of VSCode)
  # Only available if Docker is installed and running
  npx gulp test # run tests inside Docker container
  npx gulp test --grep testSuite # run only tests/suites filtered by js regex inside container
  +
  + # Alternatively, build .vsix extension and load it into VSCode for manual testing
  + yarn run vsce package --web # build vim-xxx.vsix
  ```
  ## Code Architecture
  