Update README.rst

Some instructions were out of date. I had to install `libxslt1-dev` and `build-essential` to install. I have updated the document but having to fork code for doing this is cumbersome. So I have created a [Wiki page](https://github.com/modilabs/formhub/wiki/Installation-and-Deployment) with up-to-date instructions, so that people can contribute documentation without having to fork the codebase.