Update README.md (#519)

Co-authored-by: Graham Sanderson <[email protected]>
Co-authored-by: nate contino <[email protected]>
Co-authored-by: Graham Sanderson <[email protected]>

Author: 3 people

README.md

@@ -12,10 +12,15 @@ however certain examples that use chip-specific functionality will only build on
 Similarly, Wi-Fi and Bluetooth examples will only build on a board that includes Wi-Fi and Bluetooth support.
 
 Platform and board information are passed to the CMake build via the `PICO_PLATFORM` and `PICO_BOARD` variables.
+
+By default, the Pico SDK targets builds for RP2040 (`PICO_PLATFORM=rp2040`). To build for RP2350 instead, pass
+`-DPICO_PLATFORM=rp2350` to CMake (or `-DPICO_PLATFORM=rp2350-riscv` for RISC-V). Alternatively, in many cases, you can rely
+on the board configuration to set the platform for you. For example, passing `-DPICO_BOARD=pico2` will automatically select `PICO_PLATFORM=rp2350`.
+
 For more information see the "Platform and Board Configuration" chapter of 
-the [Raspberry Pi Pico-series C/C++ SDK](https://rptl.io/pico-c-sdk) book
+the [Raspberry Pi Pico-series C/C++ SDK](https://rptl.io/pico-c-sdk) book.
 
-Information on which examples are not being built is output during the CMake configuration step.
+Information on which examples are not being built is displayed during the CMake configuration step.
 
 ### First Examples
 
@@ -46,7 +51,7 @@ App|Description
 [blink_any](binary_info/blink_any) | Uses `bi_ptr` variables to create a configurable blink binary - see the separate [README](binary_info/README.md) for more details
 [hello_anything](binary_info/hello_anything) | Uses `bi_ptr` variables to create a configurable hello_world binary - see the separate [README](binary_info/README.md) for more details
 
-### Bootloaders (RP2350 Only)
+### Bootloaders (RP235x Only)
 App|Description
 ---|---
 [enc_bootloader](bootloaders/encrypted) | A bootloader which decrypts binaries from flash into SRAM. See the separate [README](bootloaders/encrypted/README.md) for more information
@@ -66,7 +71,7 @@ App|Description
 ---|---
 [build_variants](cmake/build_variants) | Builds two version of the same app with different configurations
 
-### DCP
+### DCP (RP235x Only)
 
 App|Description
 ---|---
@@ -81,11 +86,11 @@ App|Description
 [channel_irq](dma/channel_irq) | Use an IRQ handler to reconfigure a DMA channel, in order to continuously drive data through a PIO state machine.
 [sniff_crc](dma/sniff_crc) | Use the DMA engine's 'sniff' capability to calculate a CRC32 on a data buffer.
 
-### HSTX
+### HSTX (RP235x Only)
 
 App|Description
 ---|---
-[dvi_out_hstx_encoder](dvi_out_hstx_encoder) `RP2350`| Use the HSTX to output a DVI signal with 3:3:2 RGB
+[dvi_out_hstx_encoder](hstx/dvi_out_hstx_encoder) | Use the HSTX to output a DVI signal with 3:3:2 RGB
 
 ### Flash
 
@@ -156,11 +161,11 @@ App|Description
 [multicore_runner](multicore/multicore_runner) | Set up the second core to accept, and run, any function pointer pushed into its mailbox FIFO. Push in a few pieces of code and get answers back.
 [multicore_doorbell](multicore/multicore_doorbell) | Claims two doorbells for signaling between the cores. Counts how many doorbell IRQs occur on the second core and uses doorbells to coordinate exit.
 
-### OTP
+### OTP (RP235x Only)
 
 App|Description
 ---|---
-[hello_otp](otp/hello_otp) | Demonstrate reading and writing from the OTP on RP2350, along with some of the features of OTP (error correction and page locking).
+[hello_otp](otp/hello_otp) | Demonstrate reading and writing from the OTP on RP235x, along with some of the features of OTP (error correction and page locking).
 
 ### Pico Board
 
@@ -328,7 +333,7 @@ App|Description
 [rtc_alarm](rtc/rtc_alarm) | Set an alarm on the RTC to trigger an interrupt at a date/time 5 seconds into the future.
 [rtc_alarm_repeat](rtc/rtc_alarm_repeat) | Trigger an RTC interrupt once per minute.
 
-### SHA-256
+### SHA-256 (RP235x Only)
 
 App|Description
 ---|---