add usage hints and improve README (#2)

Author: cirnatdan

README.md

@@ -1,4 +1,15 @@
-flag is a frontend for pkgsrc
+**flag** is a frontend for [pkgsrc](), a portable package build system. The goal is to
+have a tool similar to what `emerge` is for `Portage` or `yaourt` is for `AUR`.
+It should work on most platforms supported by `pkgsrc`, including NetBSD, Linux,
+macOS and others.
+
+# Usage
+**flag** wraps existing pkgsrc tools and make targets in convenient subcommands: 
+* `flag search tmux` - search for package *tmux*, equivalent of `pkgfind tmux`
+* `flag install tmux` - install package *tmux*, wrapper for `cd /usr/pkgsrc/misc/tmux && make install clean clean-depends`
+* `flag remove tmux` - wrapper for `[...] make deinstall`
+* `flag options tmux` - wrapper for `[...] make show-options`
+* `flag dependencies tmux` - wrapper for `[...] make show-depends` 
 
 # Building
 flag is developed on systems with an existing pkgsrc installation.