Notes for Apple M1 build from source (#7747)

* Create notes_for_arm.md

contrib/osx/notes_for_arm.md

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+# Notes on running Electrum from source on ARM-based Macs (Apple M1 OSX)
+
+Development version (git clone)
+
+1. Check out the code from GitHub:
+
+```
+$ git clone https://github.com/spesmilo/electrum.git
+$ cd electrum
+$ git submodule update --init
+```
+
+Run install (this should install dependencies):
+```
+python3 -m pip install --user -e .
+```
+
+2. cryptography and will need to be manually installed. It will mention pycryptodomex, but prefer cryptography:
+
+```
+$ pip install cryptography
+```
+
+3. Install libsecp256k1
+
+```
+$ contrib/make_libsecp256k1.sh
+```
+
+4. `pip install pyqt5` will work on intel x86, however for M1, to bypass pyqt5 install issue, do the following:
+
+```
+$ brew install pyqt5
+$ echo 'export PATH="/opt/homebrew/opt/qt@5/bin:$PATH"' >> ~/.zshrc
+$ echo 'export PATH="/opt/homebrew/opt/pyqt@5/5.15.4_1/bin:$PATH"' >> ~/.zshrc
+$ source ~/.zshrc
+```
+
+Finally, try it in python to ensure it works: 
+
+```
+$ python3
+>>> import PyQt5
+```
+
+5. Run electrum: 
+
+```
+$ ./run_electrum
+```
+