* [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#