Create usb.md

Create contributing (porting) API reference template for engineering.

Author: Amanda Butler

docs/reference/contributing/target/usb.md

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+### USB
+
+[Include a brief description here.]
+
+#### Assumptions
+
+##### Defined behavior
+
+[Include any defined behavior in bullet format here.]
+
+##### Undefined behavior
+
+[Include any undefined behavior in bullet format here.]
+
+##### Notes
+
+[Include any potential trouble areas in bullet format here.]
+
+#### Implementing USB
+
+[Include implementation information here.]
+
+#### Testing
+
+[Include testing information here.]