Add links to reference SD/VGA designs to README (#64)

Author: JamesH65

README.md

@@ -14,9 +14,9 @@ Library|Description
    [pico_audio_i2s](src/rp2_common/pico_audio_i2s)|Audio output via I2S on 3 GPIOs using PIO. Arbitrary frequency
    [pico_audio_pwm](src/rp2_common/pico_audio_pwm)|Audio output via (PIO) PWM. Currently a bit limited in frequency support (it was developed on FPGA to do 22050Hz at 48Mhz system clock). It does however support error diffusion dithering and noise shaping with 16x oversampling to give surprisingly good audio quality. This code will be split to provide both a fixed frequencie(s) version and a slightly slower but rather better arbitrary frequency version supporting ever higher carrier frequencies 
    [pico_audio_spdif](src/rp2_common/pico_audio_spdif)|Audio output in S/PDIF on a GPIO using PIO. Supports up to 192khz stereo. Consumed OK in test, haven't tried it with real hardware
-[pico_sd_card](src/rp2_common/pico_sd_card)|1 and 4 bit SDIO support using PIO. This is functional (currently writing is only 1 bit), but the the code is very much prototype and the API is just a placeholder - the command set needs to be separated from the SDIO and shared with SPI
+[pico_sd_card](src/rp2_common/pico_sd_card)|1 and 4 bit SDIO support using PIO. This is functional (currently writing is only 1 bit), but the code is very much a prototype and the API is just a placeholder - the command set needs to be separated from the SDIO and shared with SPI. A reference design for an SD card interface can be found in the [Hardware design with RP2040](https://datasheets.raspberrypi.com/rp2040/hardware-design-with-rp2040.pdf) datasheet.
 [pico_sleep](src/rp2_common/pico_sleep)|Low power related APIs, WIP because they are not sufficiently generic and also only handle core 0
-[pico_scanvideo](src/common/pico_scanvideo)|Support for video output where every pixel is _scanned out_ every frame. VGA/DPI support is highly functional and stable, but the API is subject to change
+[pico_scanvideo](src/common/pico_scanvideo)|Support for video output where every pixel is _scanned out_ every frame. VGA/DPI support is highly functional and stable, but the API is subject to change. A reference design for a VGA board can be found in the [Hardware design with RP2040](https://datasheets.raspberrypi.com/rp2040/hardware-design-with-rp2040.pdf) datasheet.
    [pico_scanvideo_dbi](src/rp2_common/pico_scanvideo_dbi)| currently non-compiling... placeholder for adding scanvideo over MIPI DBI support.
    [pico_scanvideo_dpi](src/rp2_common/pico_scanvideo_dpi)| Highly functional and stable support for parallel RGB output and VSYNC/HSYNC/DEN/CLOCK for VGA/DPI.
 [pico_util_buffer](src/common/pico_util_buffer)|Rather incomplete buffer abstraction, used by pico_audio and pico_scanvideo

src/common/pico_scanvideo/README.adoc

@@ -14,6 +14,8 @@ The API is currently very focused for `pico_scanvideo_dpi` which outputs paralle
 pins for DPI (via resistor DAC) VGA. In this case, whilst the monitor requires accurate timing (for VGA), the timing is fully under control of the RP2040. (note by convention color is generally
 5:5:5 RGB)
 
+There is a reference design for a VGA board based on the RP2040 in the https://datasheets.raspberrypi.com/rp2040/hardware-design-with-rp2040.pdf[Hardware design with RP2040] datasheet.
+
 `pico-scanvideo-dbi` is a (previous proof of concept - currently non-compiling) version for 16 bit MIPI DBI displays, where the data is still clocked out in parallel (though the actual timing is less critical than with signals intended for monitors). In the case of GRAM based displas, this just copies data at high speed into the GRAM, so double-buffering, or tearing support is necessary depending on the type of video your are displaying
 
 `pico-scanvideo-dvi` is not yet present, but is conceptually similar to DPI