LibGDX - make motion blur for 2D game

Newbie & Debugging Questions
#1

I made a 2D game, with basic shaders and now i wish to add motion blur, i am beginner with shaders so i have hard time to implement it.

My shaders -
Vertex shader :

attribute vec4 a_color;
    attribute vec2 a_position;
    attribute vec2 a_texCoord0;

    uniform mat4 u_projTrans;

    varying vec4 v_color;
    varying vec2 v_texCoord0;

    void main() {
	    v_color = a_color;
	    v_texCoord0 = a_texCoord0;
	    gl_Position =  u_projTrans * vec4(a_position, 0.0f, 1.0f);	
    }

Fragment shader :

 varying vec4 v_color;
    varying vec2 v_texCoord0;

    uniform vec2 screenSize;

    uniform sampler2D u_texture;
    uniform vec4 v_time;

    const float RADIUS = 0.75;

    const float SOFTNESS = 0.6;

    void main() {
    	vec4 texColor = texture2D(u_texture, v_texCoord0);
        vec4 timedColor = (v_color + v_time);

	    vec2 position = (gl_FragCoord.xy / screenSize.xy) - vec2(0.5);
	    float len = length(position);

	    float vignette = smoothstep(RADIUS, RADIUS-SOFTNESS, len);

	    texColor.rgb = mix(texColor.rgb, texColor.rgb * vignette, 0.5);

	    gl_FragColor = vec4(texColor.rgb * timedColor.rgb, texColor.a);
    }

What should I add to the shaders to create the motion blur effect?
Or maybe I can create the motion blur effect in libgdx without the shaders?

Any ideas?
Thanks in advance