* [C#] Fix and improve project files
* "net35" target included for Unity 5
* "net46" target included for Unity 2017
* "netstandard2.0" target included for Unity 2018 and general use
* "netstandard2.1" target included for Span<T> support
Included project properties for defining UNSAFE_BYTEBUFFER, BYTEBUFFER_NO_BOUNDS_CHECK, and ENABLE_SPAN_T conditional compilation symbols.
* Add documentation on building for C#
* [C#] Fix and improve project files
* "net35" target included for Unity 5
* "net46" target included for Unity 2017
* "netstandard2.0" target included for Unity 2018 and general use
* "netstandard2.1" target included for Span<T> support
Included project properties for defining UNSAFE_BYTEBUFFER, BYTEBUFFER_NO_BOUNDS_CHECK, and ENABLE_SPAN_T conditional compiler constants.
* Add documentation on building for C#
Public access to the backing buffer uses Span<T> instead of ArraySegment<T>.
Writing to the buffer now supports Span<T> in addition to T[].
To maintain backwards compatibility ENABLE_SPAN_T must be defined.
Include C# codegen in flatc and .NET FlatBuffer access via the
FlatBufferBuilder class
Tested: on Windows.
Change-Id: If5228a8df60a10e0751b245c6c64530264ea2d8a
