minor documentation issue: need to "git clone" for themisto installation instructions to work

[rohanmaddamsetti]

In the installation instructions, the first step is:
git submodule update --init --recursive

However, this does not work if I click the github link to download the latest release as a zipped directory, because .git does not exist in this case!

In order to install, I have to:
git clone https://github.com/algbio/themisto.git

and then, this works correctly:
git submodule update --init --recursive

It would be great if you could document this in the README so that others can install themisto without trouble :-)