docs: Add SGN.md page

This fixes #871

Author: boriel

docs/sgn.md

@@ -0,0 +1,34 @@
+# SGN
+
+## Syntax
+```basic
+SGN(expression)
+```
+
+## Description
+
+Returns the sign of a numeric expression as follows:
+* -1 if the number is negative
+* 0 if the number is zero
+* 1 if the number is positive
+
+The returned value type is [byte](types.md#Byte).
+
+## Examples
+
+```basic
+REM Print sign of different numbers
+PRINT "Sign of -5 is "; SGN(-5) ' Prints -1
+PRINT "Sign of 0 is "; SGN(0) ' Prints 0
+PRINT "Sign of 3.14 is "; SGN(3.14) ' Prints 1
+```
+
+## Remarks
+
+* This function is 100% Sinclair BASIC Compatible
+* If the argument is an unsigned value, the result will always be either 0 or 1
+* Using SGN with string expressions will result in a compile-time error
+
+## See also
+
+* [ABS](abs.md)