Added steps for running build_platforms.py locally

Author: evaherrada

README.md

@@ -161,3 +161,22 @@ deploy:
   on:
     tags: true
 ```
+
+## Running build_platforms.py locally
+1. Install arduino-cli from here: https://arduino.github.io/arduino-cli/installation/
+2. Download ci-arduion
+   * `git clone https://github.com/adafruit/ci-arduino`
+3. Put these lines in `.bashrc` or `.bash_profile` if you're on OSX, making sure to fill in the path to where you installed ci-arduino and replacing USER with your username.
+   * `alias test-platforms='python3 ~/path/to/ci-arduino/build_platform.py'`
+   * `export HOME=/home/USER/`
+4. Run this at the top level of the library you want to test
+   * `export GITHUB_WORKSPACE=$(pwd)`
+5. Remove everything in test library, and re-create it
+   * `rm -rf ~/Arduino/libraries/Adafruit_Test_Library/; mkdir ~/Arduino/libraries/Adafruit_Test_Library`
+6. copy current library to Adafruit_Test_Library
+   * `cp -r * ~/Arduino/libraries/Adafruit_Test_Library`
+7. Find out what boards to test. Open .github/workflows/githubci.yml and find the line that runs build_platforms.py.
+8. Run test-platforms
+   * `test-platforms main_platforms`
+   * OR, if githubci.yml specified other boards, let's say the metro m0 and pyportal:
+     * `test-platforms metro_m0 pyportal`