Link schema image to README

Author: htmlboss

README.md

@@ -11,7 +11,7 @@
 * Tool docs: [https://dfo-ocean-navigator.github.io/netcdf-timestamp-mapper/](https://dfo-ocean-navigator.github.io/netcdf-timestamp-mapper/)
 * Code Doxygen: [https://dfo-ocean-navigator.github.io/netcdf-timestamp-mapper/code-docs/index.html](https://dfo-ocean-navigator.github.io/netcdf-timestamp-mapper/code-docs/index.html)
 * Historical table schema: [https://dbdiagram.io/d/5dcc0404edf08a25543de03f](https://dbdiagram.io/d/5dcc0404edf08a25543de03f)
-
+![historical table schema](https://raw.githubusercontent.com/DFO-Ocean-Navigator/netcdf-timestamp-mapper/master/docs/img/Historical-Tables.png "Historical Table Schema")
 
 ## Building