Add README file.

README.md

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+HTTP Parser
+===========
+
+This is a parser for HTTP messages written in C. It parses both requests
+and responses. The parser is designed to be used in performance HTTP
+applications. It does not make any allocations, it does not buffer data, and
+it can be interrupted at anytime. It only requires about 120 bytes of data
+per message stream (in a web server that is per connection). 
+
+Usage
+-----
+
+One `http_parser` object is used per TCP connection. Initialize the struct
+using `http_parser_init()` and set the callbacks. That might look something
+like this:
+
+    http_parser *parser = malloc(sizeof(http_parser));
+    http_parser_init(parser, HTTP_REQUEST);
+    parser->on_path = my_path_callback;
+    parser->on_header_field = my_header_field_callback;
+    parser->data = my_socket;
+
+When data is received on the socket execute the parser and check for errors.
+
+    size_t len = 80*1024;
+    char buf[len];
+    ssize_t recved;
+
+    recved = read(fd, buf, len);
+    if (recved != 0) // handle error
+
+    http_parser_execute(parser, buf, recved);
+
+    if (http_parser_has_error(parser)) {
+      // handle error. usually just close the connection
+    }
+
+During the `http_parser_execute()` call, the callbacks set in `http_parser`
+will be executed. The parser maintains state and never looks behind, so
+buffering the data is not necessary. If you need to save certain data for
+later usage, you can do that from the callbacks. (You can also `read()` into
+a heap allocated buffer to avoid copying memory around if this fits your
+application.)
+  
+The parser decodes the transfer-encoding for both requests and responses
+transparently. That is, a chunked encoding is decoded before being sent to
+the on_body callback.
+
+It does not decode the content-encoding (gzip). Not all HTTP applications
+need to inspect the body. Decoding gzip is non-neglagable amount of
+processing (and requires making allocations). HTTP proxies using this
+parser, for example, would not want such a feature.
+