docs: added links to upgrading article

docs/sources/installation/debian.md

@@ -17,6 +17,9 @@ Description | Download
 ------------ | -------------
 Stable for Debian-based Linux | [4.2.0 (x86-64 deb)](https://s3-us-west-2.amazonaws.com/grafana-releases/release/grafana_4.2.0_amd64.deb)
 
+Read [Upgrading Grafana]({{< relref "installation/upgrading.md" >}}) for tips and guidance on updating an existing
+installation.
+
 ## Install Stable
 
 ```

docs/sources/installation/rpm.md

@@ -17,6 +17,9 @@ Description | Download
 ------------ | -------------
 Stable for CentOS / Fedora / OpenSuse / Redhat Linux | [4.2.0 (x86-64 rpm)](https://s3-us-west-2.amazonaws.com/grafana-releases/release/grafana-4.2.0-1.x86_64.rpm)
 
+Read [Upgrading Grafana]({{< relref "installation/upgrading.md" >}}) for tips and guidance on updating an existing
+installation.
+
 ## Install Stable
 
 You can install Grafana using Yum directly.

docs/sources/installation/windows.md

@@ -15,6 +15,9 @@ Description | Download
 ------------ | -------------
 Latest stable package for Windows | [grafana.4.2.0.windows-x64.zip](https://s3-us-west-2.amazonaws.com/grafana-releases/release/grafana-4.2.0.windows-x64.zip)
 
+Read [Upgrading Grafana]({{< relref "installation/upgrading.md" >}}) for tips and guidance on updating an existing
+installation.
+
 ## Configure
 
 The zip file contains a folder with the current Grafana version. Extract