Add src for extra example, update changelog

Signed-off-by: Sara Damiano <sdamiano@stroudcenter.org>

.github/workflows/build_examples.yaml

@@ -35,4 +35,5 @@ jobs:
       - name: Build PlatformIO examples
         env:
           PLATFORMIO_BUILD_FLAGS: -D SDI12_EXTERNAL_PCINT
+          PLATFORMIO_CI_SRC: examples/j_external_pcint_library
         run: pio ci --board=mayfly --board=uno --board=megaatmega2560 --board=leonardo --board=zeroUSB --board=arduino_nano_esp32 --board=feather328p --board=feather32u4 --board=adafruit_feather_m0 --board=adafruit_feather_m4 --board=adafruit_grandcentral_m4 --board=huzzah --board=featheresp32 --board=nodemcu --board=nodemcuv2 --board=esp32dev --board=esp32-c3-devkitm-1 --board=esp32-s3-devkitm-1 --project-option="lib_deps=greygnome/EnableInterrupt@^1.1.0"

ChangeLog.md

@@ -12,11 +12,18 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 ### Changed
 
 - Added python version to GitHub actions (for PlatformIO)
+- Switched to reusable workflows for GitHub actions
+- Consolidated timer prescaler math
 
 ### Added
 
+- Added support for SAMD51 processors using dedicated timers
+- Allowing (_**without testing**_) processors over 48MHz to use `micros()` function
+
 ### Removed
 
+- Offloaded some internal header file documentation to markdown files
+
 ### Fixed
 
 ***