doc(Image): Add note about indexing convention
doc(Image): Add note about indexing convention

Closes #296

thewtex/ITKBridgeJavaScript,InsightSoftwareConsortium/ITKBridgeJavaScript,thewtex/ITKBridgeJavaScript,thewtex/ITKBridgeJavaScript,InsightSoftwareConsortium/ITKBridgeJavaScript,InsightSoftwareConsortium/ITKBridgeJavaScript