Create attr.md

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Author: spectrumcomputing

docs/attr.md

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+##Requirements
+
+ATTR is a library function to be included with the following command:
+
+
+```
+#include <attr.bas>
+```
+
+##Sample usage
+
+```
+#include <attr.bas>
+
+PRINT AT 9, 10;PAPER 4; "A"
+LET s = ATTR$(9, 10)
+PRINT AT 0, 0; "The attribute of screen position 9, 10 is "; s
+```
+
+##Remarks
+
+* This function extends the one in Sinclair BASIC (and it's compatible with it) since it also allows rows 22 and 23.
+
+
+
+##See also
+
+* [ CSRLIN ](csrlin_.md)
+* [ POS](pos_.md)
+* [ AT ](at_.md)