Added zlib notice for deflate compression option in http destination docs (#248)

This is needed because zlib doesn't produce pure raw deflate data.

Author: HofiOne

doc/_admin-guide/070_Destinations/081_http/000_http_options.md

@@ -17,6 +17,9 @@ following options.
 
 *Description:* This option requests the compression of HTTP responses from the server. The available values are `gzip`, `deflate`, and `all` to enable all compression types. If no compression is required, use the `identity` value.
 
+**NOTE:** Since {{ site.product.short_name }} uses Zlib for deflate-type compressions, the `deflate` option also adds a header and footer to the compressed data. For more details consult [RFC1950](https://www.rfc-editor.org/info/rfc1950).
+{: .notice--info}
+
 ### Example: configure an http destination with compression
 
 ```config