Add instrument

Instrument.md

+# Instrument
+This element describes the instrument used to capture the Image.
+It is primarily a container for manufacturer's model and catalog
+numbers for the Microscope, LightSource, Detector, Objective and
+Filters components.
+The Objective element contains the additional elements LensNA and Magnification.
+The Filters element can be composed either of separate excitation,
+emission filters and a dichroic mirror or a single filter set.
+Within the Image itself, a reference is made to this one Filter element.
+There may be multiple light sources, detectors, objectives and filters on a microscope.
+Each of these has their own ID attribute, which can be referred to from Channel.
+It is understood that the light path configuration can be different
+for each channel, but cannot be different for each timepoint or
+each plane of an XYZ stack.