Update development installation instructions

[Roel]

From the pyOpenSci review:

There is no instructions about how to install the package for development in README file (e.g. prepare the environment: "pip install -r requirements_dev.txt", install the package in the development mode "python setup.py develop", etc)

There are bits and pieces of this information in the docs (in the installation section and the contributing section for example), but this might be hard to find and assemble.

I suggest:

• adding development installation instructions to the README
• adding the same instructions in the installation section in the docs
• add cross-reference between installation section and contributing section, maybe with a bit more info in the latter (regarding the different requirements files etc.)

[stijnvanhoey]

Basically, the dev get started boils down to...

Clone repo

git clone [email protected]:DOV-Vlaanderen/pydov.git

Setup a development environment

conda create -n pydov python=3.7
conda activate pydov

or

virtualenv pydov
source pydov/bin/activate | pydov\Scripts\activate

Install dependencies for development

Inside the environment:

pip install -e .[devs]

I would put this in the README and docs installation section and refer to the contirbuting guidelines for more in-depth information. I'm rewriting these guidelines as well to better align.

[Roel]

Closed in #274