components/expat: add expat library description

add XML instruction, see expat file.

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 four function as follows:
+
+    *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 will show them on http://expat.sourceforge.net
+
+An introductory article on using Expat is available on http://xml.com