Support editable installs

[warsaw]

Description

In the README, I see that editable installs are not support. I'm curious as to why that is, and whether you think it's an insurmountable limitation.

[sfc-gh-jfreeberg]

@domdfcoding -- Would you be open to accepting this functionality if we contributed it?

[domdfcoding]

Editable installs should work for the project you're using hatch with.

What doesn't work is specifying within requirements.txt that a dependency should be installed editable. That's what I meant by the following passage, but if that's where the confusion has arisen I can see that it could be clearer:

hatch-requirements-txt/README.rst

Lines 166 to 168 in 1692a28

	The following are unsupported:
	* Editable install commands with the ``-e`` option,

[warsaw]

That's actually a helpful distinction!