Documentation: conditional plugin specific configs

Wrap plugin specific configs into has_plugin checks to avoid breakage when Vagrantfile is shared and/or plugin is removed (for testing/development).

README.md

@@ -23,10 +23,12 @@ from within your `Vagrantfile`:
 ```ruby
 Vagrant.configure("2") do |config|
   config.vm.box = 'your-box'
+  if Vagrant.has_plugin?("vagrant-cachier")
     config.cache.auto_detect = true
     # If you are using VirtualBox, you might want to enable NFS for shared folders
     # config.cache.enable_nfs  = true
   end
+end
 ```
 
 For more information please read the documentation available at

docs/index.md

@@ -23,10 +23,12 @@ from within your `Vagrantfile`:
 ```ruby
 Vagrant.configure("2") do |config|
   config.vm.box = 'your-box'
+  if Vagrant.has_plugin?("vagrant-cachier")
     config.cache.auto_detect = true
     # If you are using VirtualBox, you might want to enable NFS for shared folders
     # config.cache.enable_nfs  = true
   end
+end
 ```
 
 For more information please check out the links on the menu above.

docs/usage.md

@@ -3,13 +3,18 @@
 ## Auto detect supported cache buckets
 
 This is the easiest way to get started with plugin. By adding the code below to
-your `Vagrantfile` you can enable automatic detection of supported cache _buckets_:
+your `Vagrantfile` you can enable automatic detection of supported cache _buckets_.
+It is a good practise to wrap plugin specific configuration with `has_plugin?` checks
+so the user's Vagrantfiles do not break if plugin is uninstalled or Vagrantfile shared
+with people not having the plugin installed.
 
 ```ruby
 Vagrant.configure("2") do |config|
   # ...
+  if Vagrant.has_plugin?("vagrant-cachier")
     config.cache.auto_detect = true
   end
+end
 ```
 
 This will make vagrant-cachier do its best to find out what is supported on the