Remove [--editable] in install instructions.

Use -e only in docs targeted at contributors.

If a user copy-pastes  command line with [--editable], they will hit
an error. If they don't know the --editable option, we're giving them
a choice to make before they can move forwards, but this isn't a choice
they need to make right now.

CONTRIBUTING.md

@@ -124,6 +124,10 @@ Follow these steps to start contributing:
    $ pip install -e .[dev]
    ```
 
+   (If transformers was already installed in the virtual environment, remove
+   it with `pip uninstall transformers` before reinstalling it in editable
+   mode with the `-e` flag.)
+
    Right now, we need an unreleased version of `isort` to avoid a
    [bug](https://github.com/timothycrosley/isort/pull/1000):