add installation instructions (#30)

Author: mathause

README.md

@@ -11,6 +11,10 @@ Without mplotutils         |  With mplotutils
 
 The code to create the examples can be found in [docs/example.py](docs/example.py).
 
+## Installation
+
+See [docs/installation.md](docs/installation.md).
+
 ## Changelog
 
 See [CHANGELOG.md](CHANGELOG.md).

docs/installation.md

@@ -0,0 +1,33 @@
+# Installation
+
+## Required dependencies
+
+- Python (3.7 or later)
+- [numpy](http://www.numpy.org/) (1.17 or later)
+- [matplotlib](http://matplotlib.org/) (3.4 or later)
+- [cartopy](http://scitools.org.uk/cartopy/) (0.17 or later)
+
+## Optional dependencies
+
+-   [xarray](http://xarray.pydata.org/) (0.15 or later)
+
+## Instructions
+
+mplotutils itself is a pure Python package, but its dependencies are not.
+The easiest way to get them installed is to use [conda](http://conda.io/) or [mamba](https://mamba.readthedocs.io/en/latest/).
+
+```bash
+mamba install -c conda-forge cartopy xarray
+```
+
+mplotutils is not available from pypi or conda-forge, therefore it needs to be installed using pip directly from github.
+
+### Install development version
+
+```bash
+pip install git+https://github.com/mathause/mplotutils
+```
+
+### Install latest released version
+
+Go to the [newest release on github](https://github.com/mathause/mplotutils/releases/latest), copy the URL of the `*.tar.gz` source file at the botton and then use pip to install it (i.e. `pip install ...`).