clarify development model: GitHub Flow

[katrinleinweber]

I noticed the following problem: what I started in #20 may too complicated, and may be obsolete once this package is released to CRAN and/or ROpenSci (#10 & #9). Then, stable version can be installed from there, so that master here could become the development version.

I could:

• remove development
• let the contributing documentation refer to https://guides.github.com/introduction/flow/