v0.2.0 release

[valentinsulzer]

Final things to do for release:

• update version number
• update version number in docs
• update authors in docs
• update changelog section title from "unreleased"

anything else? @tlestang is all the pip stuff automatic?

[valentinsulzer]

Update intro paragraph in README to:

PyBaMM (Python Battery Mathematical Modelling) solves physics-based electrochemical DAE models by using state-of-the-art automatic differentiation and numerical solvers. The Doyle-Fuller-Newman model can be solved in under 0.1 seconds, while the reduced-order Single Particle Model and Single Particle Model with electrolyte can be solved in just a few milliseconds. Additional physics can easily be included such as thermal effects, fast particle diffusion, 3D effects, and more. All models are implemented in a flexible manner, and a wide range of models and parameter sets (NCA, NMC, LiCoO2, ...) are available. There is also functionality to simulate any set of experimental instructions, such as CCCV or GITT.