add notes about limitations

Author: terrafrost

README.md

@@ -13,6 +13,18 @@ With [Composer](https://getcomposer.org/):
 $ composer require phpseclib/bcmath_compat
 ```
 
+## Limitations
+
+- `extension_loaded('bcmath')`
+bcmath_compat cannot make this return true. The recommended remediation is to not do this.
+
+- `ini_set('bcmath.scale', ...)`
+You cannot set configuration options for extensions that are not installed. If you do `ini_set('bcmath.scale', 5)` on a system without bcmath installed then `ini_get('bcmath.scale')` will return `false`. It's similar to what happens when you do `ini_set('zzz', 5)` and then `ini_get('zzz')`. You'll get `false` back.
+
+  The recommended remediation to doing `ini_set('bcmath.scale', ...)` is to do `bcscale(...)`. The recommended remediation for doing `ini_get` is (if you're using PHP >= 7.3.0) to do `bcscale()` or (if you're using PHP < 7.3.0) to do `max(0, strlen(bcadd('0', '0')) - 2)`.
+
+  Note that `ini_get` always returns a string whereas the recommended remediations return integers.
+
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