Add doc for ESP8266WebServer (#5400)

* Add doc for ESP8266WebServer

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libraries/ESP8266WebServer/README.rst

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+
+ESP8266 Web Server
+==================
+
+The WebServer class found in ``ESP8266WebServer.h`` header, is a simple web server that knows how to handle HTTP requests such as GET and POST and can only support one simultaneous client.
+
+Usage
+-----
+
+Class Constructor
+~~~~~~~~~~~~~~~~~
+
+.. code:: cpp
+
+    ESP8266WebServer server(80);
+
+Creates the ESP8266WebServer class object.
+
+*Parameters:* 
+
+host IP address: ``IPaddress addr`` (optional)
+  
+host port number: ``int port`` (default is the standard HTTP port 80)
+
+Basic Operations
+~~~~~~~~~~~~~~~~
+
+Starting the server
+^^^^^^^^^^^^^^^^^^^
+
+.. code:: cpp
+
+  void begin();
+
+Handling incoming client requests
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+.. code:: cpp
+
+  void handleClient();
+
+Disabling the server
+^^^^^^^^^^^^^^^^^^^^
+
+.. code:: cpp
+
+  void close();
+  void stop();
+
+Both methods function the same
+
+Client request handlers
+^^^^^^^^^^^^^^^^^^^^^^^
+
+.. code:: cpp
+
+  void on();
+  void addHandler();
+  void onNotFound();
+  void onFileUpload();	
+
+*Example:* 
+
+.. code:: cpp
+
+  server.on("/", handlerFunction);
+  server.onNotFound(handlerFunction); // called when handler is not assigned
+  server.onFileUpload(handlerFunction); // handle file uploads
+
+Sending responses to the client
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+.. code:: cpp
+
+  void send();
+  void send_P();
+
+*Parameters:*
+
+``code`` - HTTP response code, can be ``200`` or ``404``, etc.
+
+``content_type`` - HTTP content type, like ``"text/plain"`` or ``"image/png"``, etc.
+
+``content`` - actual content body
+
+Advanced Options
+~~~~~~~~~~~~~~~~
+
+Getting information about request arguments
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+.. code:: cpp
+
+  const String & arg();
+  const String & argName();
+  int args();
+  bool hasArg();
+
+``arg`` - get request argument value
+	
+``argName`` - get request argument name
+	
+``args`` - get arguments count
+	
+``hasArg`` - check if argument exist
+
+Getting information about request headers
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+.. code:: cpp
+
+  const String & header();
+  const String & headerName();
+  const String & hostHeader();
+  int headers();
+  bool hasHeader();
+  
+
+``header`` - get request header value
+
+``headerName`` - get request header name
+
+``hostHeader`` - get request host header if available, else empty string
+  
+``headers`` - get header count
+	
+``hasHeader`` - check if header exist
+
+Authentication
+^^^^^^^^^^^^^^
+
+.. code:: cpp
+
+  bool authenticate();
+  void requestAuthentication();
+
+``authenticate`` - server authentication, returns true if client is authenticated else false
+
+``requestAuthentication`` - sends authentication failure response to the client
+
+*Example Usage:*
+
+.. code:: cpp
+
+  if(!server.authenticate(username, password)){
+    server.requestAuthentication();
+  }
+
+
+Other Function Calls
+~~~~~~~~~~~~~~~~~~~~
+
+.. code:: cpp
+
+  const String & uri(); // get the current uri
+  HTTPMethod  method(); // get the current method 
+  WiFiClient client(); // get the current client
+  HTTPUpload & upload(); // get the current upload
+  void setContentLength(); // set content length
+  void sendHeader(); // send HTTP header
+  void sendContent(); // send content
+  void sendContent_P(); 
+  void collectHeaders(); // set the request headers to collect
+  void serveStatic();
+  size_t streamFile();
+
+For code samples enter `here <https://github.com/esp8266/Arduino/tree/master/libraries/ESP8266WebServer/examples>`__ .
+