README.md: add overview and PXE files description.

README.md

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+# tftpboot
+
+1. [Overview](#overview)
+2. [Description](#description)
+    * [PXE files](#pxe_files)
+    * [Configuration files](#configuration_files)
+    * [Preseed](#preseed)
+    * [Scripts](#scripts)
+3. [How to use it](#how_to_use_it)
+4. [TODO](#todo)
+
+## Overview
+
+All needed files to set up a PXE servers.
+
+## Description
+
+### PXE files
+Use some bootloaders files provide by :
+  * pxelinux debian package :
+    * gpxelinux.0
+    * pxelinux.0
+  * syslinux-common debian package :
+    * chain.c32
+    * menu.c32
+    * reboot.c32
+    * vesamenu.c32
+
+### Configuration files
+
+### Preseed
+
+### Script
+
+See the [scripts's README.md][scripts readme.md] file for more informations.
+
+[scripts readme.md]: https://git.ipr.univ-rennes1.fr/cellinfo/tftpboot/src/master/scripts "scripts's README.md for tftpboot"