Add Links to Contributing and Developer Guides in README

[cailyn-codes]

solves: #49

I've added a very small sentence to link to the contributing guidelines and developer guide - is this what you were thinking or would you like additional detail on how to contribute?

Note that the contributing guidelines are also linked when someone (like me!) opens their first PR on the repo. Should we avoid the duplication, or is the link useful before someone gets to the point of creating a PR?

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[rikatz]

I think adding the contrib guideline is fine as sometimes people look for that before really opening the PR

/lgtm
/approve
Thanks

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This pull-request has been approved by: cailynse, rikatz

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[astoycos]

No this is perfect, thanks @cailynse!!!

Also our README generally isn't that nice at the moment if you have time/ want to tackle something definitely feel free to!

At the very least we should be pointing folks to our website -> https://network-policy-api.sigs.k8s.io/