Add support for rendering README.rst files

[winstonolson]

Is your feature request related to a problem? Please describe.
On the EcoSML package details page (eg. https://ecosml.org/package/github/isofit/isofit), there is no support for rendering README.rst files like there is for rendering README.md files

Describe the solution you'd like
If a repository has a README.rst file at the top-level, render it on the package details page (like it is done on https://ecosml.org/package/github/EnSpec/hytools)

Describe alternatives you've considered
In the case of the isofit repository, it is not feasible to switch to markdown just for this purpose since .rst files are the current standard used for documentation.