gatemate: add README, comment on broken I2C and UART mirroring

gateware/boards/gatematea1_evb/README.md

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+Olimex GateMateA1-EVB
+---------------------
+
+As of writing (board Rev. A) has a few oddities:
+- The default dirtyJtag bootloader UART mirroring doesn't work, you have to recompile it and use the correct baud rate.
+- The 2.5V <-> 3.3V level shifter connected to Pmod1 (U6 / TXB0108) does not support I2C, external hacks are needed!
+
+
+Recompile dirtyJtag
+-------------------
+
+As of commit `938eb6d6` on upstream `https://github.com/phdussud/pico-dirtyJtag`, the only change required is to modify `cdc_uart.h` such that `USBUSART_BAUDRATE` is 1Mbaud (1000000), recompile and upload (hold RP-BOOT1 while powering up to expose a fake USB drive to drop the .uf2 file).