Malicious code and legal policies

[f-f]

In purescript/registry#76 (comment) we figured that we probably want to be able to remove "malicious code" from the registry. We also figured that NPM has already a policy on this that we might want to adopt, together with other legal policies

So we should:

• read through these policies
• port inside this repo (e.g. in a legal folder) the policies we like
• patch the README to point to the legal documents where relevant

[thomashoneyman]

Some additional legal things we need to make sure we include are described here:
purescript/registry#251 (comment)

[thomashoneyman]

Our legal advisor especially emphasized the need for DMCA protections.