replace text with link to new location

installing.md

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+Dear Pelias user,
+
+Hey there! :wave: You know how this used to be the place you could find the Pelias installation guide? 
+Well it didn't really belong here and we had to move it to another place. :grimacing:
+
+Please head over to [pelias/pelias/INSTALL.md](https://github.com/pelias/pelias/blob/master/INSTALL.md) to find it.
+It can also be viewed on [pelias.io](http://pelias.io/install.html).
+
+See you there!