doc: add USR function

Author: boriel

docs/identifier.md

@@ -103,6 +103,7 @@ Identifiers shown in bold are taken from the Sinclair BASIC (beware their meanin
 * **[THEN](if.md)**
 * **[TO](to.md)**
 * [UBOUND](ubound.md) **(function)**
+* [USR](usr.md) **(function)**
 * [UNTIL](do.md) **(statement)**
 * **[VAL](val.md)** **(function)**
 * **[VERIFY](load.md)** **(statement)**

docs/usr.md

@@ -0,0 +1,54 @@
+#USR
+
+##Syntax
+
+
+```
+USR(<address>)
+USR(<string value>)
+```
+
+##Description
+
+This function exist for the sole compatibility with Sinclair BASIC. It's not needed in ZX Basic.
+
+If used with a numeric argument, it will jump to the given memory address and start executing the machine code from there.
+To return the control to BASIC, a `ret` instruction must be executed. The value of the BC register will be used as the
+value (uInteger) returned by the function.
+
+If used with a string argument, it will return the UDG (User Defined Graphic) memory address of the first character of the string.
+For example, for the `\A` UDG, `USR "a"` will return the address of it. This function is case insensitive.
+
+Returned value type is [UInteger](types.md#Integral).
+
+##Examples
+
+To call a machine code routine:
+```
+REM Uses USR to invoke a machine code routine
+PRINT "BC register returned "; USR @myRoutine
+END
+
+myRoutine:
+Asm
+ld bc, 1234
+ret
+End Asm
+```
+
+To work with UDG:
+```
+REM Creates an UDG with Horizontal lines
+FOR i = 0 TO 7:
+    POKE USR "a" + i, 255 * (i MOD 2)
+NEXT i
+PRINT "\A is the UDG A"
+```
+
+##Remarks
+
+* This function is 100% Sinclair BASIC Compatible
+
+##See Also
+
+* [CODE](code.md)