Add references to the README.md

[pietermarsman]

The learning curve for pdf's can be quite steep. It would help new contributors (like me) if there are some recommended readings or references. I suggest we put those in the README.md.

My current list of suggestions:

• About pdf
• About font type 1

[kaushikacharya]

wiki page of the repository could be another alternate.

[jstockwin]

100% agree that some information for contributors would be great. I've moved this to "needs solution" so we can continue to discuss this.

[pietermarsman]

I almost always use the pdf reference to get the information on how stuff should be implemented. In the last year I haven't used any other resource extensively. I think this is the one most valuable resource we should get new contributors to use.

Maybe instead of adding it to the README.md we could add it to CONTRIBUTING.md?

[jstockwin]

I agree that contributing is probably the right place to add it