improved float configuration documentation (#116)

* improved float configuration documentation

* minor tweaks to floating point update

Co-authored-by: Laurence Lundblade <lgl@securitytheory.com>

README.md

@@ -201,34 +201,95 @@ unlimited.
 
 Some of the error codes have changed.
 
-## Floating Point Support
+## Floating Point Support & Configuration
+
+By default, all QCBOR floating-point features are enabled:
+
+* Encoding and decoding of basic float types, single and double-precision
+* Encoding and decoding of half-precision with conversion to/from single
+  and double-precision
+* Preferred serialization of floating-point
+* Floating point dates
+* Methods that can convert big numbers, decimal fractions and other numbers
+  to/from floating-point
+
+If full floating-point is not needed, the following #defines can be
+used to reduce object code size and dependency.
+
+See discussion in qcbor_encode.h for other details.
+
+### #define QCBOR_DISABLE_FLOAT_HW_USE
+
+This removes dependency on:
+
+* Floating-point hardware and floating-point instructions
+* `<math.h>` and `<fenv.h>`
+* The math library (libm, -lm)
+
+For most limited environments, this removes enough floating-point
+dependencies to be able to compile and run QCBOR.
+
+Note that this does not remove use of the types double and float from
+QCBOR, but it limits QCBOR's use of them to converting the encoded
+byte stream to them and copying them. Converting and copying them
+usually don't require any hardware, libraries or includes. The C
+compiler takes care of it on its own.
+
+QCBOR uses its own implementation of half-precision float-pointing
+that doesn't depend on math libraries. It uses masks and shifts
+instead. Thus, even with this define, half-precision encoding and
+decoding works.
+
+When this is defined, the QCBOR functionality lost is minimal and only
+for decoding:
+
+* Decoding floating-point format dates are not handled
+* There is no conversion between floats and integers when decoding. For
+  example, QCBORDecode_GetUInt64ConvertAll() will be unable to convert
+  to and from float-point.
+* Floats will be unconverted to double when decoding.
+
+No interfaces are disabled or removed with this define.  If input that
+requires floating-point conversion or functions are called that
+request floating-point conversion, an error code like
+`QCBOR_ERR_HW_FLOAT_DISABLED` will be returned.
+
+This saves only a small amount of object code. The primary purpose for
+defining this is to remove dependency on floating point hardware and
+libraries.
+
+### #define QCBOR_DISABLE_PREFERRED_FLOAT 
+
+This eliminates support for half-precision
+and CBOR preferred serialization by disabling
+QCBOR's shift and mask based implementation of
+half-precision floating-point.
+
+With this defined, single and double-precision floating-point
+numbers can still be encoded and decoded. Conversion
+of floating-point to and from integers, big numbers and 
+such is also supported. Floating-point dates are still
+supported. 
+
+The primary reason to define this is to save object code.
+Roughly 900 bytes are saved, though about half of this
+can be saved just by not calling any functions that
+encode floating-point numbers.
+
+### Compiler options
+
+Compilers support a number of options that control
+which float-point related code is generated. For example,
+it is usually possible to give options to the compiler to avoid all
+floating-point hardware and instructions, to use software
+and replacement libraries instead. These are usually
+bigger and slower, but these options may still be useful
+in getting QCBOR to run in some environments in 
+combination with `QCBOR_DISABLE_FLOAT_HW_USE`.
+In particular, `-mfloat-abi=soft`, disables use of 
+ hardware instructions for the float and double
+ types in C for some architectures. 
 
-By default, all QCBOR floating-point features are enabled. This includes
-encoding and decoding of half-precision, CBOR preferred serialization
-for floating-point and floating-point epoch dates.
-
-If full floating-point is not needed the following #defines can be
-used to reduce object code size.
-
-QCBOR_DISABLE_FLOAT_HW_USE -- Avoid all use of floating-point hardware
-and inclusion of <math.h> and <fenv.h>
-
-QCBOR_DISABLE_PREFERRED_FLOAT -- Eliminates support for half-precision
-and CBOR preferred serialization.
-
-Even if these are #defined, QCBOR can still encode and decode basic
-floating point numbers.
-
-Defining QCBOR_DISABLE_PREFERRED_FLOAT will reduce object code size by
-about 900 bytes, though 550 of these bytes can be avoided without the
-#define by just not calling any of the functions that encode
-floating-point numbers.
-
-Defining QCBOR_DISABLE_FLOAT_HW_USE will save a small amount of object
-code. Its main use is on CPUs that have no floating-point hardware.
-
-See discussion in qcbor_encode.h for details.
-G
 ## Comparison to TinyCBOR
 
 TinyCBOR is a popular widely used implementation. Like QCBOR,