manual/readme: To markdown, also update git repo

Author: tony

manual/README.md

@@ -0,0 +1,26 @@
+For studying the differences between prior tmux versions to check
+compatibility with legacy versions.
+
+Get source:
+
+```bash
+$ git clone https://github.com/tmux/tmux
+$ cd tmux
+```
+
+Converted with:
+
+```bash
+$ git checkout <version>
+$ ./configure
+$ make
+$ groff -t -e -mandoc -Tascii tmux.1 | col -bx > manpage.txt
+```
+
+repeat for versions.
+
+Create a git-diff style diff of version manuals:
+
+```bash
+$ diff -u 1.6 1.8 > 1_6__1_8.diff
+```