update installing doc to include homebrew instructions

docs/installing-countess/index.md

@@ -1,25 +1,50 @@
-## Installing CountESS
+# Installing CountESS
 
-CountESS can be installed from pypi:
-```
-pip install CountESS
-```
+## ... using pip
 
-... or install the latest development version directly from github:
-```
-pip install git+https://github.com/CountESS-Project/CountESS.git
-```
+CountESS can be installed from [pypi](https://pypi.org/):
 
-... or download and install for development:
-```
-git clone https://github.com/CountESS-Project/CountESS.git
-cd CountESS
-pip install -e .
-```
+    pip install countess
 
-... or run with nix:
-```
-nix run github:CountESS-Project/CountESS
-```
+... or install the latest development version directly from [github](https://github.com/):
 
+    pip install git+https://github.com/CountESS-Project/CountESS.git
 
+... or download the source and [install for development](https://pip.pypa.io/en/stable/cli/pip_install/#cmdoption-e):
+
+    git clone https://github.com/CountESS-Project/CountESS.git
+    cd CountESS
+    pip install -e .
+
+## ... using nix
+
+CountESS can be installed and run with nix:
+
+    nix run github:CountESS-Project/CountESS
+
+## ... under homebrew
+
+
+Even recent Mac installs use a very old version of Python and Tk so
+we use [homebrew](https://brew.sh/) to install a recent version.
+
+1. download the homebrew tarball & make its binaries available from your shell.
+
+   These instructions are for installing on a Mac without needing Administrator access.
+   If you already have homebrew installed you can skip this step:
+
+    mkdir bin homebrew
+    curl -L https://github.com/Homebrew/brew/tarball/master | tar xz --strip 1 -C homebrew
+    echo 'export PATH=$HOME/bin:$HOME/homebrew/bin:$PATH' >> .bash_profile
+    source .bash_profile
+
+2. Use homebrew to install a recent Tk and Python 3.10 into your `$HOME/homebrew/bin`
+   directory.  This step is pretty slow as it has to compile stuff:
+
+    brew install tcl-tk python@3.10 python-tk@3.10
+    ln -s $HOME/homebrew/bin/python3.10 bin/python
+    ln -s $HOME/homebrew/bin/pip3.10 bin/pip
+
+3. Install countess using pip:
+
+    pip install countess