Merge pull request #149 from HarperDB/load-component-env-vars

add env loading to component docs

Author: kriszyp

docs/developers/components/built-in.md

@@ -7,6 +7,7 @@ Harper provides extended features using built-in components. They do **not** nee
 	- [graphql](#graphql)
 	- [graphqlSchema](#graphqlschema)
 	- [jsResource](#jsresource)
+	- [loadEnv](#loadenv)
 	- [rest](#rest)
 	- [roles](#roles)
 	- [static](#static)
@@ -66,6 +67,29 @@ jsResource:
   files: './resource.js'
 ```
 
+## loadEnv
+
+Load environment variables via files like `.env`.
+
+This component is a [Resource Extension](./reference.md#resource-extension) and can be configured with the [`files`, `path`, and `root`](./reference.md#resource-extension-configuration) configuration options.
+
+Ensure this component is specified first in `config.yaml` so that environment variables are loaded prior to loading any other components.
+
+```yaml
+loadEnv:
+  files: '.env'
+```
+
+This component matches the default behavior of dotenv where existing variables take precedence. Specify the `override` option in order to override existing environment variables assigned to `process.env`:
+
+```yaml
+loadEnv:
+  files: '.env'
+  override: true
+```
+
+> Important: Harper is a single process application. Environment variables are loaded onto `process.env` and will be shared throughout all Harper components. This means environment variables loaded by one component will be available on other components (as long as the components are loaded in the correct order).
+
 <!-- ## login -->
 
 <!-- ## mqtt -->