Add CGO_LDFLAGS_ALLOW to the installation guide

docs/installation.md

@@ -48,13 +48,13 @@ $ git push heroku master
   $ sudo apt-get install libvips-dev
 
   # FreeBSD
-  pkg install -y pkgconf vips
+  $ pkg install -y pkgconf vips
   ```
 
 2. Next, install imgproxy itself:
 
   ```bash
-  $ go get -f -u github.com/DarthSim/imgproxy
+  $ CGO_LDFLAGS_ALLOW="-s|-w" go get -f -u github.com/DarthSim/imgproxy
   ```
 
 **Note:** imgproxy build is automatically tested with the latest three minor versions of Go and libvips. Successfully build with the older versions is not guaranteed but yet may be possible.