add instruments section

instruments/README.md

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+# instruments
+
+a directory of ready-to-use instruments.
+these are organized in the following categories:
+
+- **FM**: FM instruments for OPN/OPM.
+- **OPL**: FM instruments for OPL.
+- **OPLL**: FM instruments for OPLL.
+- **other**: instruments for any other system.
+
+# submit instruments
+
+you may submit your own instruments by creating a pull request or contacting me!
+
+the only guideline is that I (strongly) prefer original instruments.
+
+thanks for your contribution!