Add example in documentation about how to warm/hot-start in C

[darnstrom]

[AeroTH310]

I was about to ask this question myself. Right now, I'm having to infer an awful lot from the code itself, but it is still not clear to me whether a warm start feature is implemented at all. If it is, can you just give me a rough outline showing how to use it? Then I will infer the rest. Actually, I may even write your example for you and PR it.