Various recommendations

[DSotnikov]

• Added various How To Prevent recommendations to A3, A4, A6, A7, A8, A9.
• Added a scenario of admin level gained by a parameter change in A5 exploit scenario.
• Added APISecurity.io as education resource in What's Next for Developers.
• Changed the git repo clone URL in contributing.md.

[PauloASilva]

Hi @DSotnikov,
Thanks for your contribution.

I had the chance to review your PR.
Please check my comments.

If you agree with my comments I can do the required changes and merge your PR.

As per the CONTRIBUTING.md, Pull Requests should be opened to the develop branch.
I can solve this locally, adding your suggestions to the develop branch. They should be released very soon.

Cheers,
Paulo A. Silva

[DSotnikov]

Thanks, @PauloASilva, most changes you propose are great. A3 is the only one in which I think my original suggestion was hard to understand and so the meaning got lost. See my new proposal.

And yes, feel free to make the changes and merge the PR as appropriate.

Thanks a lot for your help and your work on the project!

[PauloASilva]

Hi @DSotnikov,
Your contributions were added and are available in v0.1.2.

Thanks for contributing,
Paulo A. Silva