QRACK_FPPOW env. var. in README

README.md

@@ -27,6 +27,8 @@ where `c` is a PyZX circuit object. The circuit will automatically be simulated
 
 See [https://pyqrack.readthedocs.io/en/latest/](https://pyqrack.readthedocs.io/en/latest/) for an API reference.
 
+For custom Qrack build floating-point precision, where options are `half`, `float`, `double`, and `quad`, set an environment variable via `export QRACK_FPPOW=[n]` (or as appropriate to your shell) where `[n]` is the logarithm base 2 of the number of bits in the systemic floating point type (`4`, `5`, `6`, or `7`, with `5` or `float` as default, i.e. `2**5=32` for 32-bit `float`).
+
 Please feel welcome to open an issue, if you'd like help. 😃
 
 **Special thanks go to Zeeshan Ahmed, for bug fixes and design suggestions, Ashish Panigrahi, for documentation and design suggestions, WingCode, for documentation, and to the broader community of Qrack contributors, for years of happy Qracking! You rock!**