Plugin json (#474)

* json encoder

* thread to pass messages

* description for example

crates/nu_plugin_example/src/main.rs

@@ -1,6 +1,30 @@
-use nu_plugin::serve_plugin;
+use nu_plugin::{serve_plugin, CapnpSerializer};
 use nu_plugin_example::Example;
 
 fn main() {
-    serve_plugin(&mut Example {})
+    // When defining your plugin, you can select the Serializer that could be
+    // used to encode and decode the messages. The available options are
+    // CapnpSerializer and JsonSerializer. Both are defined in the serializer
+    // folder in nu-plugin.
+    serve_plugin(&mut Example {}, CapnpSerializer {})
+
+    // Note
+    // When creating plugins in other languages one needs to consider how a plugin
+    // is added and used in nushell.
+    // The steps are:
+    // - The plugin is register. In this stage nushell calls the binary file of
+    //      the plugin sending information using the encoded PluginCall::Signature object.
+    //      Use this encoded data in your plugin to design the logic that will return
+    //      the encoded signatures.
+    //      Nushell is expecting and encoded PluginResponse::Signature with all the
+    //      plugin signatures
+    // - When calling the plugin, nushell sends to the binary file the encoded
+    //      PluginCall::CallInfo which has all the call information, such as the
+    //      values of the arguments, the name of the signature called and the input
+    //      from the pipeline.
+    //      Use this data to design your plugin login and to create the value that
+    //      will be sent to nushell
+    //      Nushell expects an encoded PluginResponse::Value from the plugin
+    // - If an error needs to be sent back to nushell, one can encode PluginResponse::Error.
+    //      This is a labeled error that nushell can format for pretty printing
 }