Better showcase package installation command (#361)

This patch updates the README to better showcase the package
installation commands. Most people probably just want to install
everything and it probably won't hurt if they do so. So make that the
default and then add a clarifying explanation and what is what rather
than telling them to install more packages below.

README.md

@@ -72,17 +72,16 @@ pip3 install pipenv
 
 The actual dependencies:
 ```shell
-pipenv sync --system
+pipenv sync --system --categories "packages dev-packages ci"
 ```
 Note that the above command will only work from the root of the repository
 since it needs to have `Pipfile.lock` in the working directory at the time
 of execution.
 
-If you plan on doing development work, make sure you grab the development
-and CI categories of packages as well:
-```shell
-pipenv sync --system --categories "dev-packages ci"
-```
+The above command will also install all the packages, including development
+packages (the `dev-packages` category), and packages only needed in CI (the
+`ci` category). If you do not need those, you can omit them from the categories
+option.
 
 Optionally, to run tests (run_tests.sh), you also need: