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`.

README.md

@@ -82,9 +82,12 @@ so make sure that is on your :code:`PATH` variable.
 
 ### Development version (git clone)
 
+_(For OS-specific instructions, see [here for Windows](contrib/build-wine/README_windows.md),
+and [for macOS](contrib/osx/README_macos.md))_
+
 Check out the code from GitHub:
 ```
-$ git clone git://github.com/spesmilo/electrum.git
+$ git clone https://github.com/spesmilo/electrum.git
 $ cd electrum
 $ git submodule update --init
 ```

contrib/build-wine/README_windows.md

@@ -0,0 +1,61 @@
+# Running Electrum from source on Windows (development version)
+
+## Prerequisites
+
+- [python3](https://www.python.org/)
+- [git](https://gitforwindows.org/)
+
+## 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 ".[gui,crypto]"
+```
+
+### 2. Install `libsecp256k1`
+
+[libsecp256k1](https://github.com/bitcoin-core/secp256k1) is a required dependency.
+This is a C library, which you need to compile yourself.
+Electrum needs a dll, named `libsecp256k1-0.dll`, placed into the inner `electrum/` folder.
+
+For Unix-like systems, the (`contrib/make_libsecp256k1.sh`) script does this for you,
+however it does not work on Windows.
+If you have access to a Linux machine (e.g. VM) or perhaps even using
+WSL (Windows Subsystem for Linux), you can cross-compile from there to Windows,
+and build this dll:
+```
+$ GCC_TRIPLET_HOST="x86_64-w64-mingw32" ./contrib/make_libsecp256k1.sh
+```
+
+Alternatively, MSYS2 and MinGW-w64 can be used directly on Windows, as follows.
+
+- download and install [MSYS2](https://www.msys2.org/)
+- run MSYS2
+- inside the MSYS2 shell:
+  ```
+  $ pacman -Syu
+  $ pacman -S --needed git base-devel mingw-w64-x86_64-toolchain mingw-w64-x86_64-autotools
+  $ export PATH="$PATH:/mingw64/bin"
+  ```
+  `cd` into the git clone, e.g. `C:\wspace\electrum` (auto-mounted at `/c/wspace/electrum`)
+  ```
+  $ cd /c/wspace/electrum
+  $ GCC_TRIPLET_HOST="x86_64-w64-mingw32" ./contrib/make_libsecp256k1.sh
+  ```
+
+(note: this is a bit cumbersome, see [issue #5976](https://github.com/spesmilo/electrum/issues/5976)
+for discussion)
+
+### 3. Run electrum: 
+
+```
+> python3 ./run_electrum
+```
+

contrib/osx/notes_for_arm.md → contrib/osx/README_macos.md

@@ -1,9 +1,14 @@
-# Notes on running Electrum from source on ARM-based Macs (Apple M1 OSX)
+# Running Electrum from source on macOS (development version)
 
-Development version (git clone)
+## Prerequisites
 
-1. Check out the code from GitHub:
+- [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
@@ -15,15 +20,21 @@ Run install (this should install most dependencies):
 $ python3 -m pip install --user -e ".[crypto]"
 ```
 
-2. Install libsecp256k1
-
+### 2. Install libsecp256k1
 ```
+$ brew install autoconf automake libtool coreutils
 $ contrib/make_libsecp256k1.sh
 ```
 
-3. `pip install pyqt5` would work on intel x86, however there are no prebuilt wheels on PyPI for M1.
-As a workaround, we can install it from brew:
+### 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
@@ -31,16 +42,7 @@ $ echo 'export PATH="/opt/homebrew/opt/pyqt@5/5.15.4_1/bin:$PATH"' >> ~/.zshrc
 $ source ~/.zshrc
 ```
 
-Try it in python to ensure it works: 
-
-```
-$ python3
->>> import PyQt5
-```
-
-4. Run electrum: 
-
+### 4. Run electrum:
 ```
 $ ./run_electrum
 ```
-