Added a note in README.md for new contributors

[Butt4cak3]

We're receiving an increasing number of PRs of people trying to submit entirely new chapters. Even though @leios decided that he would allow some people to write new chapters, it's not an entirely open-to-the-public process yet.

This PR adds a very visible note right at the top of the README.md file in the root of this repository. I think a note like this is long overdue. New contributors cannot know the current situation when it comes to chapter contributions. With this note, we can at least point to something when we close such PRs.

[leios]

Yeah, this is a good idea. As a note, the following statement is already in the How to Contribute page: "At this point, I am trying to figure out the best way to balance community contributions and text. Right now, I feel comfortable writing the text associated with each algorithm and asking for the community to write individual implementations. In the future, I might allow other users to write algorithm chapters, but for now let's keep it simple: I'll do the writing, everyone else does the coding."

I thought that would be enough. I'll allow chapter submissions if people talk to me about it first.

[Butt4cak3]

@leios The thing is that it's just not visible enough in my opinion. Otherwise we wouldn't have this number of chapter pull requests right now. But if this doesn't help, then I don't know what does.

[leios]

This is a fair warning. I think it should be accepted.

[Butt4cak3]

We can always remove it when we feel it's not necessary anymore.