Create external plugins doc file and update contributing guide (#8001)

CONTRIBUTING.md

@@ -14,9 +14,11 @@
 1. Open a new [pull request][].
 
 #### Contributing an External Plugin *(experimental)*
-Input plugins written for internal Telegraf can be run as externally-compiled plugins through the [Execd Input Plugin](https://github.com/influxdata/telegraf/tree/master/plugins/inputs/execd) without having to change the plugin code.
+Input, output, and processor plugins written for internal Telegraf can be run as externally-compiled plugins through the [Execd Input](plugins/inputs/execd), [Execd Output](/plugins/inputs/execd), and [Execd Processor](plugins/processors/execd) Plugins without having to change the plugin code.
+
+Follow the guidelines of how to integrate your plugin with the [Execd Go Shim](/plugins/common/shim) to easily compile it as a separate app and run it with the respective `execd` plugin. 
+Check out our [guidelines](docs/EXTERNAL_PLUGINS.md#external-plugin-guidelines) on how to build and set up your external plugins to run with `execd`.
 
-Follow the guidelines of how to integrate your plugin with the [Execd Go Shim](https://github.com/influxdata/telegraf/tree/master/plugins/inputs/execd/shim) to easily compile it as a separate app and run it from the inputs.execd plugin. 
 
 #### Security Vulnerability Reporting
 InfluxData takes security and our users' trust very seriously. If you believe you have found a security issue in any of our

docs/EXTERNAL_PLUGINS.md

@@ -0,0 +1,60 @@
+### External Plugins
+
+External plugins are external programs that are built outside of Telegraf that
+can run through an `execd` plugin. These external plugins allow for more flexibility
+compared to internal Telegraf plugins.  
+
+- External plugins can be written in any language (internal Telegraf plugins can only written in Go)
+- External plugins can access to libraries not written in Go
+- Utilize licensed software that isn't available to the open source community
+- Can include large dependencies that would otherwise bloat Telegraf
+
+### External Plugin Guidelines
+The guidelines of writing external plugins would follow those for our general [input](docs/INPUTS.md), 
+[output](docs/OUTPUTS.md), [processor](docs/PROCESSORS.md), and [aggregator](docs/AGGREGATOR.md) plugins. 
+Please reference the documentation on how to create these plugins written in Go.
+
+
+#### Execd Go Shim
+For Go plugins, there is a [Execd Go Shim](plugins/common/shim) that will make it trivial to extract an internal input, processor, or output plugin from the main Telegraf repo out to a stand-alone repo.  This shim This allows anyone to build and run it as a separate app using one of the `execd`plugins:
+- [inputs.execd](/plugins/inputs/execd)
+- [processors.execd](/plugins/processors/execd)
+- [outputs.execd](/plugins/outputs/execd)
+
+Follow the [Steps to externalize a plugin](plugins/common/shim#steps-to-externalize-a-plugin) and [Steps to build and run your plugin](plugins/common/shim#steps-to-build-and-run-your-plugin) to properly with the Execd Go Shim
+
+#### Step-by-Step guidelines
+This is a guide to help you set up your plugin to use it with `execd`
+1. Write your Telegraf plugin.  Depending on the plugin, follow the guidelines on how to create the plugin itself using InfluxData's best practices:
+   - [Input Plugins](/docs/INPUTS.md)
+   - [Processor Plugins](/docs/PROCESSORS.md)
+   - [Aggregator Plugins](/docs/AGGREGATORS.md)
+   - [Output Plugins](docs/OUTPUTS.md)
+2. If your plugin is written in Go, include the steps for the [Execd Go Shim](plugins/common/shim#steps-to-build-and-run-your-plugin)
+  1. Move the project to an external repo, it's recommended to preserve the path
+  structure, (but not strictly necessary). eg if your plugin was at
+  `plugins/inputs/cpu`, it's recommended that it also be under `plugins/inputs/cpu`
+  in the new repo. For a further example of what this might look like, take a
+  look at [ssoroka/rand](https://github.com/ssoroka/rand) or
+  [danielnelson/telegraf-execd-openvpn](https://github.com/danielnelson//telegraf-execd-openvpn)
+  1. Copy [main.go](./example/cmd/main.go) into your project under the `cmd` folder.
+  This will be the entrypoint to the plugin when run as a stand-alone program, and
+  it will call the shim code for you to make that happen. It's recommended to
+  have only one plugin per repo, as the shim is not designed to run multiple
+  plugins at the same time (it would vastly complicate things).
+  1. Edit the main.go file to import your plugin. Within Telegraf this would have
+  been done in an all.go file, but here we don't split the two apart, and the change
+  just goes in the top of main.go. If you skip this step, your plugin will do nothing.
+  eg: `_ "github.com/me/my-plugin-telegraf/plugins/inputs/cpu"`
+  1. Optionally add a [plugin.conf](./example/cmd/plugin.conf) for configuration
+  specific to your plugin. Note that this config file **must be separate from the
+  rest of the config for Telegraf, and must not be in a shared directory where
+  Telegraf is expecting to load all configs**. If Telegraf reads this config file
+  it will not know which plugin it relates to. Telegraf instead uses an execd config
+  block to look for this plugin. 
+ 
+
+
+
+
+