Added document for plantuml (#3763)

docs/layers/lang/plantuml.md

@@ -11,6 +11,8 @@ description: "This layer is for PlantUML development, syntax highlighting for Pl
 - [Features](#features)
 - [Install](#install)
   - [Layer](#layer)
+- [Configuration](#configuration)
+  - [Limitation](#limitation)
 
 <!-- vim-markdown-toc -->
 
@@ -33,3 +35,21 @@ To use this configuration layer, update custom configuration file with:
 [[layers]]
   name = "lang#plantuml"
 ```
+
+## Configuration
+
+When you want to see picture which plantuml draw, you need to set
+plantuml_jar_path. You should find or install plantuml.jar in 
+your system.
+Then you give the path to `plantuml_jar_path`. see below.
+
+This is just example.
+```toml
+  plantuml_jar_path = "/usr/share/plantuml/plantuml.jar"
+```
+
+### Limitation
+
+You can't do any opration While you preview the result of plantuml.
+You write one more part of plantuml diagram, you can not see all
+picures. You can see first one.