add external plugins in docs

docs/EXTERNAL_PLUGINS.md

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+### 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
+To-be-added
+
+
+
+