added diff for PCB, non electrical changes

PMC_REVB/FTRN_PMC_REVB_PCB/Changes.txt

+This document describes changes on the PCB compared to REVA that are not mentioned 
+on the first page of the REVB Schematic:
+
+- AUX1 connector is moved towards the front panel edge to not interfere with the VME64x connector on the VME carrier
+
+- AUX1 connector is "sinked" into the PCB similarly as HEX switches to decrease height 
+  on the top side of the board and stay inside allowed envelope (this was issue on the carriers
+  where all four PMC headers were mounted)
+
+- USB connecors for USB microcontrollers and JTAG are moved towards the center of the board for about 0.4mm.
+  Previously connector metal was more than 1mm over the edge of the board and this could 
+  interfere with the neigbouring board in case one (or two) FTRNs would be mounted on PMC carrier with
+  two slots where gap between PMC boards is 1mm.
+
+- fixed DSH1 silkscreen (diode direction)
+
+- SFP cage stays in the same position, but prior to mounting one of the cage pins will have to be removed
+  because it interferes with front panel metal
+  
\ No newline at end of file