Added section for note about rp2040_boot2

When not using rp-pico but rp2040_boot2
the bootloader needs added. Added a section
about this in the readme.

README.md

@@ -20,6 +20,7 @@ If you aren't using a debugger (or want to use cargo-embed/probe-rs-debugger), c
     <li><a href="#installation-of-development-dependencies">Installation of development dependencies</a></li>
     <li><a href="#running">Running</a></li>
     <li><a href="#alternative-runners">Alternative runners</a></li>
+    <li><a href="#notes-on-using-rp2040_boot2">Notes on using rp2040_boot2</a></li>
     <li><a href="#roadmap">Roadmap</a></li>
     <li><a href="#contributing">Contributing</a></li>
     <li><a href="#code-of-conduct">Code of conduct</a></li>
@@ -231,6 +232,23 @@ Some of the options for your `runner` are listed below:
 
 </details>
 
+</details>
+<!-- Notes on using rp2040_hal and rp2040_boot2 -->
+<details open="open">
+  <summary><h2 style="display: inline-block" id="notes-on-using-rp2040_boot2">Notes on using rp2040_boot2</h2></summary>
+
+  When rp2040_boot2 is used instead of rp-pico the bootloader
+  must be written to the .boot2 sector. This can be done by adding the
+  following to the beginning of main.rs:
+  ```rust
+  use rp2040_boot2;
+  #[link_section = ".boot2"]
+  #[used]
+  pub static BOOT_LOADER: [u8; 256] = rp2040_boot2::BOOT_LOADER_W25Q080;
+  ```
+
+</details>
+
 <!-- ROADMAP -->
 
 ## Roadmap