[tepl] readme: Markdown format, update and improve

[Sébastien Wilmet <swilmet src gnome org>]

commit d027a6aca3ae581d5ef36fdb4dec6711ab01650e
Author: Sébastien Wilmet <swilmet gnome org>
Date:   Fri Nov 6 08:42:42 2020 +0100

    readme: Markdown format, update and improve

 README    | 54 ------------------------------------------------------
 README.md | 43 +++++++++++++++++++++++++++++++++++++++++++
 2 files changed, 43 insertions(+), 54 deletions(-)
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diff --git a/README.md b/README.md
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+Tepl - Text editor product line
+===============================
+
+This is version 5.1.2 of Tepl.
+
+Tepl is a library that eases the development of GtkSourceView-based text
+editors and IDEs.
+
+Tepl was previously named Gtef (GTK text editor framework). The project has
+been renamed in June 2017 to have a more beautiful name. The end of Tepl is
+pronounced like in “apple”.
+
+The Tepl library is free software and is released under the terms of the GNU
+Lesser General Public License, see the `LICENSES/` directory for more
+information.
+
+See also the [Tepl web page](https://wiki.gnome.org/Projects/Tepl).
+
+Dependencies
+------------
+
+- GLib >= 2.64
+- GTK >= 3.22
+- GtkSourceView >= 4.0
+- [Amtk](https://wiki.gnome.org/Projects/Amtk) >= 5.0
+- [ICU](http://site.icu-project.org/)
+
+Installation
+------------
+
+To build the latest version of Tepl plus its dependencies from Git,
+[Jhbuild](https://wiki.gnome.org/Projects/Jhbuild) is recommended.
+
+Documentation
+-------------
+
+See the `gtk_doc` Meson option. A convenient way to read the API documentation
+is with the [Devhelp](https://wiki.gnome.org/Apps/Devhelp) application.
+
+How to contribute
+-----------------
+
+See the file `HACKING`.