README: add Windows- & macOS-specific "how to run from source" readmes

note: unclear where to put these files... `contrib/build-wine/` and `contrib/osx/`
contain binary-building-related files. maybe we could have a `doc/` folder
if the need for more similar files arise.
atm there are these two, plus maybe `contrib/docker_notes.md`.

contrib/osx/README_macos.md

@@ -0,0 +1,48 @@
+# Running Electrum from source on macOS (development version)
+
+## Prerequisites
+
+- [brew](https://brew.sh/)
+- python3
+- git
+
+## Main steps
+
+### 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 most dependencies):
+```
+$ python3 -m pip install --user -e ".[crypto]"
+```
+
+### 2. Install libsecp256k1
+```
+$ brew install autoconf automake libtool coreutils
+$ contrib/make_libsecp256k1.sh
+```
+
+### 3. Install PyQt5
+
+On Intel-based (x86_64) Macs:
+```
+$ python3 -m pip install --user pyqt5
+```
+
+Re ARM-based Macs (Apple M1), there are no prebuilt wheels on PyPI.
+As a workaround, we can install it from `brew`:
+```
+$ 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
+```
+
+### 4. Run electrum:
+```
+$ ./run_electrum
+```