qmlgl: add README.md with information on building for non-linux platforms with qmake

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+# Building for non-linux platforms
+
+Compiling the gstqmlgl plugin for non-linux platforms is not so trivial.
+This file explains the steps that need to be followed for a successful build.
+
+## Step 1
+
+Build GStreamer for the target platform using cerbero.
+
+## Step 2
+
+Enter the cerbero shell:
+```
+./cerbero-uninstalled -c config/<target platform config>.cbc shell
+```
+
+## Step 3
+
+Export the following environment variables:
+```
+export PATH=/path/to/Qt/<version>/<platform>/bin:$PATH
+```
+
+if you are cross-compiling (ex. for android), also export:
+```
+export PKG_CONFIG_SYSROOT_DIR=/
+```
+
+Additionally, if you are building for android:
+```
+export ANDROID_NDK_ROOT=$ANDROID_NDK
+```
+
+**Note**: the ANDROID_NDK variable is set by the cerbero shell; if you are not
+using this shell, set it to the directory where you have installed the android
+NDK. Additionally, if you are not building through the cerbero shell, it is also
+important to have set PKG_CONFIG_LIBDIR to $GSTREAMER_ROOT/lib/pkgconfig.
+
+## Step 4
+
+cd to the directory of the gstqmlgl plugin and run:
+```
+qmake .
+make
+```
+
+## Step 5
+
+Copy the built plugin to your $GSTREAMER_ROOT/lib/gstreamer-1.0 or link to it
+directly if it is compiled statically