Make awesome-kubectl-plugins available via krew custom-indexex

[ishantanu]

As mentioned in this comment custom-indexes in #4, we can now add custom indexes to krew and they can be used to install plugins in a similar way as default krew index.

This opens up an interesting prospect of making awesome-kubectl-plugins available to krew as a custom index. Users will be able to install plugins from kubectl-plugins directly via krew.