Documentation for C bindings

docs/source/CUsage.md

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+Use in C    {#flatbuffers_guide_use_c}
+==========
+
+The C language binding exists in a separate project named [FlatCC](https://github.com/dvidelabs/flatcc).
+
+The `flatcc` C schema compiler can generate code offline as well as
+online via a C library. It can also generate buffer verifiers and fast
+JSON parsers, printers.
+
+Great effort has been made to ensure compatibily with the main `flatc`
+project.
+
+
+## General Documention
+
+- [Tutorial](@ref flatbuffers_guide_tutorial) - select C as language
+  when scrolling down
+- General Use in C (the README) <https://github.com/dvidelabs/flatcc/blob/master/README.md>
+- The C Builder Interface, advanced <https://github.com/dvidelabs/flatcc/blob/master/doc/builder.md>
+
+
+## Basic Reflection
+
+The C-API does support reading binary schema (.bfbs)
+files via code generated from the `reflection.fbs` schema, and an
+[example usage](https://github.com/dvidelabs/flatcc/tree/master/samples/reflection)
+shows how to use this. The schema files are pre-generated
+in the [runtime distribution](https://github.com/dvidelabs/flatcc/tree/master/include/flatcc/reflection). Extended reflection
+
+
+## Mutating Reflection
+
+The C-API does not support mutating reflection like C++ does.
+
+Although the following isn't reflection, it is possible to create new
+buffers using complex objects from existing buffers as source. This can
+be very efficient due to direct copy semantics without endian conversion or
+temporary stack allocation.
+
+It is currently not possible to use an existing table or vector of table
+as source, but it would be possible to add support for this at some
+point.
+
+
+## Why not integrate with the `flatc` tool?
+
+[It was considered how the C code generator could be integrated into the
+`flatc` tool](https://github.com/dvidelabs/flatcc/issues/1), but it
+would either require that the standalone C implementation of the schema
+compiler was dropped, or it would lead to excessive code duplication, or
+a complicated intermediate representation would have to be invented.
+Neither of these alternatives are very attractive, and it isn't a big
+deal to use the `flatcc` tool instead of `flatc` given that the
+FlatBuffers C runtime library needs to be made available regardless.