Twenty Nineteen: Include contributing.txt file with Sass compiler instructions

[kjellr]

In the GitHub repository, there is a CONTRIBUTING.md document with instructions for how to compile the Sass for Twenty Nineteen. This file was not migrated over to core.

For those working with the theme (both for direct contributions, and for their own purposes), it'd be helpful to bundle instructions for working with the Sass via the included build tool. The attached patch ports over the "Compiling SCSS" section from the document on GitHub, as a new contributing.txt file.

​https://github.com/WordPress/twentynineteen/blob/master/CONTRIBUTING.md#compling-scss

[nielslange]

@kjellr I made some minor changes, such as changing my-compter to my-computer Apparently, no one noticed this typo yet. 😁

I've also mentioned running grunt build or grunt watch in the WordPress root folder, to compile the changes from /src into /build.

[kjellr]

@nielslange Thanks for fixing that typo. 😄

As for the other note:

Please note that you also need to run grunt build or grunt watch within /my-computer/local-wordpress-install/ to compile the changes from /my-computer/local-wordpress-install/src to /my-computer/local-wordpress-install/build.

This may be confusing for some people, as they may be working from a downloaded copy of the theme. I'd either note that this only applies when working directly with core, or eliminate it (it's more of a general core development convention than a theme workflow note anyway).

[nielslange]

Sounds good, @kjellr. Feel free to remove my notes again. 😉

[kjellr]

Added 46108-2.patch, which removes that line about grunt-build & grunt-watch.