Render obsolete the 'master' branch

'main' has been the primary branch since December 2020 when the project
underwent a major refactoring that brought about the unification of the
two themes into a single code base.

The packages 'modus-operandi-theme' and 'modus-vivendi-theme' were
deleted from GNU ELPA yesterday (see Emacs bug#49574), following a ~3
month period during which they would unconditionally pull in the new
'modus-themes' package.

This formally fixes issue 174 by Damien Cassou:
<https://gitlab.com/protesilaos/modus-themes/-/issues/174>.

README.md

@@ -1,29 +1,12 @@
 # Modus themes (Modus Operandi and Modus Vivendi)
 
-A pair of highly accessible themes that conform with the WCAG AAA
-standard for colour contrast between background and foreground colour
-combinations.
+1. The `master` branch is obsolete.  **Switch to `main`**.
+2. The old packages, `modus-operandi-theme` and `modus-vivendi-theme`
+   have been discontinued.  **Switch to `modus-themes`**.
+3. A summary of all new customisation options is available here:
+   <https://protesilaos.com/modus-themes/#h:bf1c82f2-46c7-4eb2-ad00-dd11fdd8b53f>.
+   Or evaluate this form to read the Info manual: 
    
-The themes are built into GNU Emacs (Emacs' git `master` branch, aka
-version `28.0.50`).  They are also distributed in several packages
-formats.
-
-+ 'modus-operandi' is light
-+ 'modus-vivendi' is dark
-
-## Further information
-
-Read the [Info manual HTML](https://protesilaos.com/modus-themes)
-version for how to install, load, enable, and customise the themes.
-
-If you are using the latest version of the themes from this repository's
-`master` branch (e.g. the MELPA packages) or version >= `0.13.0`, the
-manual is already installed on your machine: evaluate `(info
-"(modus-themes) Top")` to start reading it.
-
-The themes cover a lot of packages and are highly customisable.
-
-For some demo content, check:
-
-+ the screenshots https://protesilaos.com/modus-themes-pictures/
-+ my videos https://protesilaos.com/code-casts/
+   ```
+   (info "(modus-themes) Customization Options")
+   ```