'readme' changes

README.md

 # HTTP File Upload plugin for libpurple (XMPP Protocol)
 Experimental XEP-0363: HTTP File Upload plugin for libpurple (Pidgin, Finch, etc.)
 
+## Installation
+### Linux, macOS
+1. Install the dependencies (`libpurple-dev`, `libglib2.0-dev`, `libgio2.0-dev`)
+2. `git clone https://github.com/Junker/purple-xmpp-http-upload.git`
+3. `cd purple-xmpp-http-upload`
+4. `make`
+5. A final `make install` should copy the plugin into your libpurple plugin dir.
+