Add main page documentation
* - <a href="https://github.com/MaJerle/ESP_AT_Lib">Official development repository on Github</a>
* - <a href="http://www.espressif.com/sites/default/files/documentation/4a-esp8266_at_instruction_set_en.pdf">Official AT commands instruction set by Espressif systems</a>
*
+ * \section sect_contribute How to contribute
+ *
+ * - Official development repository is hosted on Github
+ * - <a href="https://github.com/MaJerle/c_code_style">Respect C style and coding rules</a>
+ *
* \section sect_license License
*
* \verbatim
