Update beginners doc to install directly from github

[dshean]

4. Install pygeotools and demcoreg from latest github source:

• python -m pip install git+https://github.com/dshean/pygeotools.git
• python -m pip install git+https://github.com/dshean/demcoreg.git
• python -m pip install git+https://github.com/dshean/imview.git

Requires some reworking of PATH and ability to call scripts.