Merge branch 'feature/instruct_nghttp_xml' into 'master'

Feature/instruct nghttp xml

add expat and nghttp library description, instruct the XML parse and HTTP/2 protocol.

See merge request !119

components/expat/expat.rst

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+The Expat XML Parse Instruction
+=============================
+
+Expat is an XML parser library written in C which be used for parse XML documents.
+
+It is a stream-oriented parser in which an application registers handlers for things the parser might find in the XML document.
+
+It can parse some larger files.
+
+-   Expat XML Parser support many different processor, but for the most part function you only need the following functions:
+
+    **XML_ParserCreate**: Create a new parser object
+
+    **XML_SetElementHandler**: Set handlers for start and end tags
+
+    **XML_SetCharacterDataHandler**: Set handler for text
+
+    **XML_Parse**: Pass a buffer full of document to the parser
+
+More information about Expat library can be found on http://expat.sourceforge.net
+
+An introductory article on using Expat is available on http://xml.com

components/nghttp/nghttp.rst

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+The nghttp Instruction
+=============================
+
+This is an implementation of the Hypertext Transfer Protocol version 2 in C.
+
+The framing layer of HTTP/2 is implemented as a reusable C library.
+
+An HPACK encoder and decoder are available as a public API.
+
+-   The nghttp support many different processor, but for the most part you only need the following functions:
+
+    **nghttp2_session_callbacks_new**: Initializes \*callbacks_ptr with NULL values
+
+    **nghttp2_session_client_new**: Initializes \*session_ptr for client use
+
+    **nghttp2_session_callbacks_del**: Frees any resources allocated for callbacks
+
+    **nghttp2_submit_settings**: Stores local settings and submits SETTINGS frame
+
+    **nghttp2_submit_request**: Submits HEADERS frame and optionally one or more DATA frames
+
+    **nghttp2_session_want_read**: Returns nonzero value if session wants to receive data from the remote peer
+
+    **nghttp2_session_want_write**: Returns nonzero value if session wants to send data to the remote peer
+
+    **nghttp2_session_recv**: Receives frames from the remote peer
+
+    **nghttp2_session_send**: Sends pending frames to the remote peer
+
+    **nghttp2_session_del**: Frees any resources allocated for session
+
+If you are following TLS related RFC, you know that NPN is not the standardized way to negotiate HTTP/2. NPN itself is not even published as RFC. 
+
+The standard way to negotiate HTTP/2 is ALPN, Application-Layer Protocol Negotiation Extension, defined in RFC 7301. 
+
+-   The following endpoints are available to try out the nghttp2 implementation:
+
+    https://nghttp2.org/ (TLS + ALPN/NPN)
+
+    This endpoint supports h2, h2-16, h2-14, spdy/3.1 and http/1.1 via ALPN/NPN and requires TLSv1.2 for HTTP/2 connection.
+
+More information about nghttp library can be found at https://nghttp2.org
+
+An introductory article on protocol is available on RFC 7540 HTTP/2 and RFC 7541 HPACK - Header Compression for HTTP/2