New documentation

[FSMaxB]

The goal of this pull request is to rewrite the documentation in a more structured way that can also be somewhat used as a reference to look something up and contains all the gotchas.

Although the current style of "letter to the user" has its appeal, the kind of questions I have been getting over the time and that appeared on StackOverFlow make me think that a different approach should be taken.

Feel free to take a look at this work in progress and give some feedback.

[FSMaxB]

TODO:

• Explain the data structure
• Explain parsing
• Explain printing
• Explain how to work with the data structure (adding/removing items to/from arrays/objects etc.)
• Full example
• More information about building on Windows
• Other things

[PerMalmberg]

This is a great improvement of the old, rather sparse, documentation.

[FSMaxB]

@PerMalmberg Good to hear, that's exactly what I was going for.

[FSMaxB]

While writing the documentation I noticed the horrible cJSON_Add macros (what Dave called "auto mode"). I intend to replace them with functions that return booleans instead of deprecating them in the documentation. (this is what is still blocking me from 1.7 at the moment.)

[PerMalmberg]

@FSMaxB I'll take functions instead of macros any day. 👍

[FSMaxB]

Done! Release 1.7 will probably come tomorrow or as a new years gift.