Adding documentation for readRenderTargetPixels

Discovered while investigating #7680

docs/api/renderers/WebGLRenderer.html

@@ -227,6 +227,9 @@
 		<div>If forceClear is true, the depth, stencil and color buffers will be cleared before rendering even if the renderer's autoClear property is false.</div>
 		<div>Even with forceClear set to true you can prevent certain buffers being cleared by setting either the .autoClearColor, .autoClearStencil or .autoClearDepth properties to false.</div>
 
+		<h3>[method:null readRenderTargetPixels]( [page:WebGLRenderTarget renderTarget], [page:Float x], [page:Float y], [page:Float width], [page:Float height], buffer )</h3>
+		<div>Reads the pixel data from the renderTarget into the buffer you pass in. Buffer should be a Javascript Uint8Array instantiated with new Uint8Array( renderTargetWidth * renderTargetWidth * 4 ) to account for size and color information. This is a wrapper around gl.readPixels.</div>
+
 		<h3>[method:null renderImmediateObject]( camera, lights, fog, material, object )</h3>
 		<div>Renders an immediate Object using a camera.</div>