FEAT: modernize README

[jorgepiloto]

The goal of this pull-request is to fix #610. However, I decided to go one step further and update as much as possible our README.md file by:

• Using the svg logo as the banner image for the repository. It appears now as a centered image.
• Transpose the metadata table and add a Maintainers header pointing to a new AUTHORS.md file.
• Replace the molniya plot by a nice collage of examples.
• Enhanced the installation guide with a table covering conda, pypi and source installation.
• Add a table in the Examples, background and talks pointing to our talks in conferences.
• Updated license copyright years.

Everything is pure markdown, which esures that PyPI will render properly. Please, take a look to:

https://github.com/jorgepiloto/poliastro/tree/fix/molniya-plot

Before I fix the link to the image for the collage of examples when merging.

[astrojuanlu]

Do you think we could use interactive=True for this one? 😄

[jorgepiloto]

You mean having an interactive plot directly in README.md?

[astrojuanlu]

Don't think we can have an interactive plot in the README, but rather taking a screenshot of how the Plotly plot looks like, given that we're favoring OrbitPlotter2D/3D in the docs and the examples over StaticOrbitPlotter.

[jorgepiloto]

Just updated the plot by imposing the interactive = True parameter 👍🏽

[astrojuanlu]

Thanks a lot @jorgepiloto! My only comment is that the screenshot is very wide, and it doesn't look very good in the docs:

Wondering if there's a way to shrink it a bit, or otherwise leave the legend out completely and crop the screenshot so that only the axis are shown.

[astrojuanlu]

(Request changes)

[jorgepiloto]

We could render that plot and make it dynamic in the docs by using:

.. jupyter-execute::
   :hide-code:

   from poliastro.examples import molniya
   molniya.plot(interactive=True)

If not, I will fix the size 👍🏽

[astrojuanlu]

That would be cool indeed! We'll need a static plot for the README anyway

[astrojuanlu]

Fantastic job @jorgepiloto! Left a few comments

[jorgepiloto]

It looks like the latest version of matplotlib is causing the porkchop features to fail. Let me fix that in a separate pull-request.