How to use cpp11eigen?

1. Start a new project with RStudio.
2. Open `dev/01_load_or_install.R` and run it.
3. Open `src/01_ols.cpp`, inspect the OLS functions.
4. Create your own functions in `02_your_functions.cpp`.
5. Run `devtools::load_all()` in `dev/01_load_or_install.R` to test your
   functions.
6. Run `dev/03_readme_and_license.R` to add a README and license.
7. Run `devtools::install()` in `dev/01_load_or_install.R` to install the
   package locally when you are ready.
