Mention HHVM precision issue in readme

Author: Riimu

README.md

@@ -243,13 +243,23 @@ apply to following calls.
 
 ## Known Issues ##
 
-If you are running this library on a PHP version earlier than 5.4.5, you may
-receive the following error, if you are trying to encode a recursive array:
+### Recursive arrays on PHP < 5.4.5 ###
+
+If you try to encode recursive arrays on PHP versions earlier than 5.4.5, you
+may encounter the following error (due to the way array comparisons work
+internally in PHP):
 
 `Fatal error:  Nesting level too deep - recursive dependency?`
 
-In order to fix this, it is recommended to disable `recursion.detect` and set
-a max value for `recursion.max` if you expect to encounter recursive arrays.
+In order to fix this, you should disable the recursive array detection by
+setting the option `recursion.detect` to false and set a value for the option
+`recursion.max` to prevent recursive arrays from causing an infinite loop.
+
+### Float precision on HHVM ###
+
+Note that HHVM does not support the ini setting `serialize_precision`. Thus,
+the use of the value `false` for the option `float.precision` is not supported
+on HHVM.
 
 ## Credits ##