docs: add doc for RCall

README.md

@@ -234,6 +234,26 @@ Results:
 1 study.huhao.demo.domain.core.common.Page.getConvertedContent
 ```
 
+### Reverse Call Graph
+
+```
+coca rcall -c org.bytedeco.javacpp.tools.TokenIndexer.get
+```
+
+Results:
+
+```
+digraph G { 
+edge [dir="back"];
+
+"org.bytedeco.javacpp.tools.Parser.extern" -> "org.bytedeco.javacpp.tools.Parser.declarations";
+"org.bytedeco.javacpp.tools.Parser.declarations" -> "org.bytedeco.javacpp.tools.Parser.extern";
+...
+}
+```
+
+![RCall Demo](docs/sample/rcall.svg)
+
 ### Auto Refactor
 
 support: 

core/domain/call_graph/call_graph.go

@@ -22,7 +22,9 @@ func (c CallGraph) Analysis(funcName string, clzs []models.JClassNode) string {
 	return dotContent
 }
 
+// TODO: be a utils
 func ToGraphviz(chain string) string {
+	//rankdir = LR;
 	var result = "digraph G { \n"
 	result = result + chain
 	result = result + "}\n"