Docs: Adjust the docs to run with build script

Point users to use the build script to build
the ffmpeg plugin instead of manually building it.

Use the versions for ffmpeg from versions.env

Author: DawidWesierski4

ecosystem/ffmpeg_plugin/README.md

@@ -6,8 +6,26 @@ FFmpeg is an open source project licensed under LGPL and GPL. See <https://www.f
 
 ## 1. Build
 
+You can use the ./build.sh that will build the script for you automatically,
+
+``` bash
+apt-get install wget patch unzip
+# in the  ecosystem/ffmpeg_plugin/ folder
+./build.sh
+```
+
+The manual way of building is described below
+
 ### 1.1. Build openh264
 
+Before building, check if openh264 is already installed:
+
+```bash
+ldconfig -p | grep libopenh264
+```
+
+If the command outputs a path to `libopenh264.so`, openh264 is present and you can skip the build step below.
+
 ```bash
 git clone https://github.com/cisco/openh264.git
 cd openh264
@@ -18,27 +36,30 @@ sudo ldconfig
 cd ../
 ```
 
+
 ### 1.2. Build FFmpeg with MTL patches
 
 > Note: `$mtl_source_code` should be pointed to top source code tree of Media Transport Library.
 
 ```bash
+. $mtl_source_code/versions.env
+
 git clone https://github.com/FFmpeg/FFmpeg.git
 cd FFmpeg
-git checkout release/7.0
+git checkout release/${FFMPEG_VERSION}
 # apply the build patch
-git am $mtl_source_code/ecosystem/ffmpeg_plugin/7.0/*.patch
+git am $mtl_source_code/ecosystem/ffmpeg_plugin/${FFMPEG_VERSION}/*.patch
 # copy the mtl in/out implementation code
 cp $mtl_source_code/ecosystem/ffmpeg_plugin/mtl_*.c -rf libavdevice/
 cp $mtl_source_code/ecosystem/ffmpeg_plugin/mtl_*.h -rf libavdevice/
 # build with --enable-mtl, customize the option as your setup
 ./configure --enable-shared --disable-static --enable-nonfree --enable-pic --enable-gpl --enable-libopenh264 --enable-encoder=libopenh264 --enable-mtl
-make -j "$(nproc)"
+make -j
 sudo make install
 sudo ldconfig
 ```
 
-> Note: For FFmpeg version 4.4 or 6.1, replace 7.0 with 4.4 or 6.1 respectively in above commands.
+> Note: For FFmpeg version 4.4 or 6.1, export `FFMPEG_VERSION=4.4` or 6.1.
 
 In order to build the FFmpeg in a non-default path, use `--prefix="<path>"` within the `./configure` command to point to an empty installation folder.