Add comment to install "t" and disable "todo.plugin" (#1742)

This commit is intended to help new uses of the
modern-t theme get it set up properly
and avoid the "todo.plugin" breaking it

Please see the following issues for details.
https://github.com/Bash-it/bash-it/issues/1693
https://github.com/Bash-it/bash-it/issues/1374

Co-authored-by: Russell Adams <russell.adams@avast.com>

themes/modern-t/modern-t.theme.bash

@@ -1,3 +1,10 @@
+# The "modern-t" theme is a "modern" theme variant with support
+# for "t", the minimalist python todo list utility by Steve Losh.
+# Get and install "t" at https://github.com/sjl/t#installing-t
+#
+# Warning: The Bash-it plugin "todo.plugin" breaks the "t"
+# prompt integration, please disable it while using this theme.
+
 SCM_THEME_PROMPT_PREFIX=""
 SCM_THEME_PROMPT_SUFFIX=""