Added Usage section

README.md

@@ -17,3 +17,39 @@ git submodule update --init --recursive
 ./configure
 make
 ```
+## Usage
+
+### Sending text message
+Here is a simple example of how to send text message:
+```cpp
+#include <telebotxx/BotApi.hpp>
+
+std::string token = "YOUR:BOT:TOKEN";
+int main()
+{
+  using namespace telebotxx;
+  BotApi bot(token);
+  Message answer = bot.sendMessage(ChatId{"@chat_name"},
+                                   Text{"Hello, world!"}
+                                  );
+  return 0;
+}
+```
+Member function ```BotApi::sendMessage()``` returns information about sent message on success or throws an exception.
+
+You can pass additional options in any combination and order:
+```cpp
+int messageId = ...; // ID of the original message to reply
+Message answer = bot.sendMessage(ChatId{"@chat_name"},
+                                 Text{"Hello, world!"},
+                                 ReplyTo{messageId},
+                                 ParseMode::Markdown,
+                                 DisableNotification(),
+                                 DisableWebPagePreview()                                 
+                                );
+```
+Note, that ```ChatId``` argument can be specified with its name or id:
+```cpp
+ChatId{"@chat_name"}
+ChatId{123456}
+```