Quick README update. (#41)

Will likely need updating again but this closes #5.

README.md

@@ -2,10 +2,16 @@
 
 A Python library for petro-elastic modelling. It contains a `Material` class for representing rocks and fluids, as well as various rock physics models and algorithms such as Hashin-Shtrikman bounds and Gassmann fluid substitution.
 
+[![Build and test](https://github.com/equinor/open_petro_elastic/actions/workflows/python-build-test.yml/badge.svg)](https://github.com/equinor/open_petro_elastic/actions/workflows/python-build-test.yml)
+[![Build documentation](https://github.com/equinor/open_petro_elastic/actions/workflows/python-sphinx-doc.yml/badge.svg)](https://github.com/equinor/open_petro_elastic/actions/workflows/python-sphinx-doc.yml)
+[![PyPI version](https://img.shields.io/pypi/v/open_petro_elastic.svg)](https://pypi.org/project/open_petro_elastic//)
+[![PyPI versions](https://img.shields.io/pypi/pyversions/open_petro_elastic.svg)](https://pypi.org/project/open_petro_elastic//)
+[![PyPI license](https://img.shields.io/pypi/l/open_petro_elastic.svg)](https://pypi.org/project/open_petro_elastic/)
+
 
 ## Installation
 
-```
+```shell
 pip install open_petro_elastic
 ```
 
@@ -16,9 +22,20 @@ Developers and contributors can download the repository and do `pip install ".[d
 
 The tool can be used as a library, or from the command line using [YAML](https://en.wikipedia.org/wiki/YAML) configuration files. For example, see the [tutorial config file](tutorial_config/tutorial_config.yaml).
 
-Get help on the command line interface:
+Generally, the `open_petro_elastic.material` module is used for interfacing with `open_petro_elastic` as a Python library:
 
+```python
+>>> from open_petro_elastic.material import Material
+>>> from open_petro_elastic.material.sandstone import hertz_mindlin
+>>> mineral = Material(bulk_modulus=1e9, shear_modulus=1e9, density=1000)
+>>> sand = hertz_mindlin(mineral, porosity=0.4, pressure=1e6)
+>>> print(sand.density)
+600.0
 ```
+
+To get help on the command line interface:
+
+```shell
 open_petro_elastic --help
 ```