Add README.md to make it easier for all Git forges

Thanks to Dave Abrahams for asking me to put a video demo in the
README.  This inspired me to update this repository to be in the style
of the others I also maintain.

Dave's comment was made in issue 12 on the GitHub mirror:
<#12>.

README.md

@@ -0,0 +1,20 @@
+# Logos (logos.el)
+
+This package provides a simple approach to setting up a "focus mode"
+for Emacs.  It uses the `page-delimiter` (typically the `^L`
+character) or the `outline-regexp` together with some commands to move
+between pages whether narrowing is in effect or not.  It also provides
+some optional stylistic tweaks which come into effect when the
+buffer-local `logos-focus-mode` is enabled.  The manual shows how to
+extend the code to achieve the desired result.
+
++ Package name (GNU ELPA): `logos`
++ Official manual: <https://protesilaos.com/emacs/logos>
++ Change log: <https://protesilaos.com/emacs/logos-changelog>
++ Git repo on SourceHut: <https://git.sr.ht/~protesilaos/logos>
+  - Mirrors:
+    + GitHub: <https://github.com/protesilaos/logos>
+    + GitLab: <https://gitlab.com/protesilaos/logos>
++ Mailing list: <https://lists.sr.ht/~protesilaos/logos>
++ Video demo: <https://protesilaos.com/codelog/2022-03-11-emacs-logos-demo/>
++ Backronyms: `^L` Only Generates Ostensible Slides; Logos Optionally Goes through Outline Sections

README.org

@@ -623,8 +623,8 @@ Logos is meant to be a collective effort.  Every bit of help matters.
   Omar Antolín Camarena, Philip Kaludercic, Remco van 't Veer, and user
   Ypot.
 
-+ Ideas and/or user feedback :: Daniel Mendler, Marcel Ventosa,
-  Xiaoduan, Ypot.
++ Ideas and/or user feedback :: Daniel Mendler, Dave Abrahams, Marcel
+  Ventosa, Xiaoduan, Ypot.
 
 * GNU Free Documentation License
 :PROPERTIES: