markdown: move plugins documentation

markdown/How-do-I-use-it.md

@@ -21,6 +21,7 @@
 - **[How do I use it?](#how-do-i-use-it)**
   * [Mostly-Unified CLI](#mostly-unified-cli)
   * [Flow Layout](#flow-layout)
+  * [Plugins](#plugins)
   * [Caveats & Oddities](#caveats--oddities)
 
 ## How do I use it?
@@ -167,6 +168,27 @@ All `times` related files will reside in `${DOCKER_FINANCE_CONTAINER_FLOW}/times
 
 See the container `times help` command for details.
 
+### Plugins
+
+Plugins allow you to use docker-finance public APIs, libraries and environment (client and/or container) to meet your unique needs. These plugins are categorical; as in, there are client-side ("custom") plugins and repository ("repo") plugins. Additionally, there are subcategories such as `docker`, `finance` and `root` (respective to their modules).
+
+Client-side custom plugins allow you to drop-in any code that you write and keep them locally. Repository plugins are plugins that remain within the repository and will require a pull request for any changes to be made. Client-side custom plugins can be used for either client or container modules (see directory layout).
+
+Upon client `gen`, a client-side directory layout is generated. This layout consists of:
+  - `${DOCKER_FINANCE_CLIENT_PLUGINS}/client/docker`
+    - Custom plugins that function only client-side (`lib_docker`)
+  - `${DOCKER_FINANCE_CLIENT_PLUGINS}/container/{finance,root}`
+    - Custom plugins that function only container-side (`lib_finance`, `root`)
+
+**WARNING: don't change the parent client-side directory layout** (although, you can add subdirectories):
+  - e.g., `${DOCKER_FINANCE_CLIENT_PLUGINS}/container/finance/my_experimental_plugins/{file1.ext,file2.ext}`
+
+This client-side "custom" layout somewhat mirrors the repository's plugins layout (see [`client/plugins`](https://gitea.evergreencrypto.co/EvergreenCrypto/docker-finance/src/branch/master/client/plugins) and [`container/plugins`](https://gitea.evergreencrypto.co/EvergreenCrypto/docker-finance/src/branch/master/container/plugins)).
+
+For more information, see the example plugins and help usage of each module, e.g.; `plugins help` or `help()`.
+
+> Note: for custom plugins within directory `docker` and `finance` that utilize the shell, any language can be used so long as the file is executable, reads the shell environment and can initiailize their respective libraries (`lib_docker`, `lib_finance`).
+
 ### Caveats & Oddities
 
 #### Caveats & Oddities: Flow