Documented another easy way to build scikit-image for virtualenv users.

Some of us like to work on projects in virtualenvs. Typically you may want to install/run/use the dev version of scikit-image in a virtualenv. And you may like to use pip -- it's convenient. So `requirements.txt` gives the build dependencies, you just run `$ pip install -r requirements.txt` as I documented in `DEPENDS.txt`

Apparently there is no proper standard for naming files à la `requirements.txt` (http://www.pip-installer.org/en/latest/requirements.html). I'm not trying to say everyone should use virtualenv + pip (some people have discussed the role of `requirements.txt`: https://github.com/jordansissel/fpm/issues/308). Right now the `requirements.txt` file contains the build dependencies.  Maybe later we can add commented lines for other dependencies, or have other files such as `requirements-test.txt` which would include `nose`...

This PR doesn't add informational value.  I just think that having a `requirements.txt` file will make things look familiar and straightforward to pip users.  It's just an additional way to have things work out of the box.