Readme: Add advice about dist/ and git-fu

[skip-ci]

Author: joallard

README.md

@@ -85,10 +85,15 @@ grunt --plugins=remove_button,restore_on_backspace
 ```
 
 ### Contributing
+When issuing a pull request:
 
-When issuing a pull request, *please exclude changes in the "dist"
-folder to avoid merge conflicts*. Please include tests with your feature
-so that we're not tempted to break it in the future!
+* please **do not include/commit changes in the `dist/` folder** to avoid
+  merge conflicts.  A good way to include the right files is to use
+  `git gui` or `git add` when committing to select the files you want to
+  add to your commit.
+
+* please **include tests** with your feature so that we're not tempted to
+  break it in the future!
 
 Add an entry to the top of the CHANGELOG, and update the documentation
 in `docs/` as needed. (Refactors and documentation changes don't need a
@@ -99,6 +104,13 @@ with a concise and descriptive message. One commit means one
 feature/bugfix/thing that has changed, or a diff bringing the code one
 step forward to a better, working state.
 
+Once your commit is nice and clean, and you want to *discard the other
+changes*, you can use `git checkout .` (that will erase changes to
+tracked files) and `git clean [-i/--interactive]` (to erase untracked
+files).  **However, be careful with those commands, as their function
+is to erase things/changes.**
+
+
 #### Tests
 Please ensure all the tests pass:
 
@@ -118,6 +130,10 @@ $ grunt serve
 
 You can then run the examples in `http://localhost:8000/examples/`.
 
+However, be careful not to add the `dist/` files in your commit, as
+Grunt automatically regenerates the files in `dist/` as the source is
+changed.
+
 ## License
 
 Copyright © 2013–2016 [Brian Reavis](http://twitter.com/brianreavis) & [Contributors](https://github.com/selectize/selectize.js/graphs/contributors)