Updated Postgis async (markdown)

Postgis-async.md

@@ -1 +1,29 @@
-TODO - part of https://github.com/mapnik/mapnik/pull/2010
+The PostGIS pluging can be used asynchrnonously since Mapnik 2.3 which can reduce map rendering time if you have several PostGIS layers.
+
+## How it works
+Mapnik uses the painter's algorithm to render maps. It means that layers are drawn sequentially. The inner algorithmer is :
+    For each layer
+       Query the features from the layer
+       For each feature in this layer
+           Draw the layer
+
+In this case, the renderer spends a lot of time waiting for PostGis to perform the query that will feed with features.
+
+The asynchronous_request parameter in PostGIS pulgin aims to parallelize querries on the remote server and rendering : while a layer is rendering, queries are sent ahead.
+
+
+## When to use it
+Must have :
+* You have already applyed all the [rendering optimizations with PoistGIS](/wiki:OptimizeRenderingWithPostGIS/)
+* you have a lot of PostGIS layers
+Nice to have :
+* rendering time for layers are quite homogeneous
+* your PostGIS database is on another server
+
+## When not to use it
+* you use cache-features=false ; if you want to reduce map rendering time, you should not send the same data base query twice if you have enough RAM to store the query restults
+* you have less thant 3 PostGIS layers
+* you have very heterogenous layers : for example, a huge road layers that takes  times longer to render than the other layers
+
+## How to use it
+