Add README upload support

[mairin]

We need README upload support in the backend to enable this in the frontend.

[EUGINELETHAL]

@mairin could you expound more on this issue?

[mairin]

@EUGINELETHAL Sure, and maybe @jennydaman or @jbernal0019 might be able to explain a bit better too - basically, we wanted to be able to show READMEs files for individual plugins uploaded to the ChRIS store, but the ChRIS store backend doesn't supporting uploading README files for a plugin, so methods have to be added to the backend to allow the frontend client to upload & retrieve a README file to a given plugin in the store. One of the reasons this is important is that if the backend is storing the README, it can store the README per version of the plugin, so if the README file changes upstream, we will still have an oldre version of the README in storage that is accurate for the version of the plugin it is attached too (a little bit more on this is in this ticket: FNNDSC/ChRIS_store_ui#128)

This is one of the mockups where you can see where the README would be used in the frontend:

FNNDSC/ChRIS_store_ui#81 (comment)