DOC: Update installation instructions (#241)
DOC: Update installation instructions (#241)

Installation instructions are simplified and updated to:

1. Include details on installing the C/C++ compilers from conda-forge (for development versions)
2. Add instructions for updating the development version code or switching branches
3. Remove PyPI installation instructions because it's strongly discouraged and not officially supported.
4. Remove references to Anaconda environments - most users needing these installation instructions for pycalphad are well supported by just using the base environment


* Update installation instructions to include development version details and SymEngine
* Don't create a new environment, just install to the base conda environment

tkphd/pycalphad,tkphd/pycalphad,tkphd/pycalphad