Update README.md

Author: John-Colvin

README.md

@@ -1,4 +1,21 @@
 NetCDF-D
 ========
 
-D bindings to the NetCDF library
+D bindings to the NetCDF C library (https://github.com/Unidata/netcdf-c and http://www.unidata.ucar.edu/software/netcdf/)
+
+Usage
+-----
+
+```import netcdf;``` and you're away.
+
+Everything should work identically to the C version, with the exception of the backward compatibility and deprecated sections which have been removed. D version blocks have been used in place of C version macros. The names of the versions are kept the same.
+
+
+Note that this is a binding a C library, where type sizes are somewhat implementation defined. I would recommend against using functions taking ```c_long```/```c_ulong``` in favour of ```int```/```uint``` and ```long```/```ulong```.[1]
+
+Architectures / C compilers where C types don't comply with ```sizeof(int) == 4 && sizeof(long long) == 8``` or have ```float```/```double``` types laid out differently to the IEEE-754 standard single and double precision formats are not supported at this time.[2]
+
+
+[1] Note that the function naming has not been changed. Functions with ```long```/```ulong``` in the name take ```c_long```/```c_ulong``` as parameters, functions with (u)longlong in the name take ```long```/```ulong```.
+
+[2] Things will probably still work if you can avoid using the parts of the C API that use the offending types.