readme update

Author: jacobwilliams

README.md

@@ -67,6 +67,20 @@ To generate the documentation using [FORD](https://github.com/Fortran-FOSS-Progr
   ford numdiff.md
 ```
 
+By default, the library is built with double precision (`real64`) real values. Explicitly specifying the real kind can be done using the following processor flags:
+
+Preprocessor flag | Kind  | Number of bytes
+----------------- | ----- | ---------------
+`REAL32`  | `real(kind=real32)`  | 4
+`REAL64`  | `real(kind=real64)`  | 8
+`REAL128` | `real(kind=real128)` | 16
+
+For example, to build a single precision version of the library, use:
+
+```
+fpm build --profile release --flag "-DREAL32"
+```
+
 ## Documentation
 
 The latest API documentation can be found [here](https://jacobwilliams.github.io/NumDiff/). This was generated from the source code using [FORD](https://github.com/Fortran-FOSS-Programmers/ford) (note that the included `build.sh` script will also generate these files).