Extend dev guide with dependency handling recommondations

[cehbrecht]

We should extend the developer guide with recommendations for conda/pip dependency handling.

The birdhouse chat for conda:

My current strategy is to pin packages in the environment.yml only if necessary. Use a cron-job at travis-ci to see if things break. In addition I try to make use of the conda spec files, see an example mentioned here:
cp4cds/c4cds-wps#25

The owslib issues:
geopython/OWSLib#565

[nilshempelmann]

status so far:
https://birdhouse.readthedocs.io/en/latest/dev_guide.html#developer-guidelines