DOC: Advertise soft-matter dev channel in README.

When we make an important change to the master branch and we want our collaborators to be able to install the bleeding-edge version without git or pip headaches, we can roll fresh conda builds and put them into soft-matter/dev. Then they just:

    conda config --add channels soft-matter  # only necessary to do this once
    conda install -c soft-matter/channel/dev trackpy

I will include said conda recipes into the repo in time. For the moment there's some hacking related to the 0.2.3/0.2.4 release quirks, but dev builds will be straightforward in the future.