Add STR documentation

Author: boriel

docs/str.md

@@ -0,0 +1,32 @@
+#STR
+
+##Syntax
+
+
+```
+STR(<numeric value>)
+```
+
+##Description
+
+Converts the given numeric value to a String. It's the opposite of [VAL](val.md)
+Returned value type is [String](types.md#UByte).
+
+##Examples
+
+```
+REM Convert numeric expression to value
+LET a = -5.2
+PRINT STR(a + 1)
+```
+
+##Remarks
+
+* This function is 100% Sinclair BASIC Compatible
+
+##See Also
+
+* [CHR](chr.md)
+* [CODE](code.md)  
+* [STR](str.md)
+* [VAL](chr.md)