INSTALL: add markdown syntax

Reviewed-by: llogan
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

INSTALL → INSTALL.md

+#Installing FFmpeg:
 
-1) Type './configure' to create the configuration. A list of configure
-options is printed by running 'configure --help'.
+1. Type `./configure` to create the configuration. A list of configure
+options is printed by running `configure --help`.
 
-'configure' can be launched from a directory different from the FFmpeg
+    `configure` can be launched from a directory different from the FFmpeg
 sources to build the objects out of tree. To do this, use an absolute
-path when launching 'configure', e.g. '/ffmpegdir/ffmpeg/configure'.
+path when launching `configure`, e.g. `/ffmpegdir/ffmpeg/configure`.
 
-2) Then type 'make' to build FFmpeg. GNU Make 3.81 or later is required.
+2. Then type `make` to build FFmpeg. GNU Make 3.81 or later is required.
 
-3) Type 'make install' to install all binaries and libraries you built.
+3. Type `make install` to install all binaries and libraries you built.
 
 NOTICE
+------
 
  - Non system dependencies (e.g. libx264, libvpx) are disabled by default.