Add a metadate store for additional information (extra text in readme for example)

[rajbos]

Use case: we want to change the docker image being used, so we do that on a new branch in the forked repo, to prevent the changes from being overwritten when updating the fork.

We want that warning / label / additional information for this repo visible in the marketplace, so that our users are aware they need to change the way they use this action. In this example, none of the branches of the parent will work, since the runners will not be able to pull in containers from public registries.

• define a data structure for this 'markup'
• load that data structure in the front end
• display the data / label on the marketplace, maybe on the (to be created) detail page as a banner on top of the readme