README: Add basic build directions

README: Add basic build directions

In particular, highlight the use of configure flags to control locating
X11 keyboard stuff when building for Wayland.

Of particular note, if the locale root is not specified, then xkbcommon
will look for them under $prefix (i.e. /usr/local/share/X11/locale).
But unless the user has specifically installed them there, it is better
to look in the standard system location, /usr/share/X11/locale.

Otherwise, xkbcommon will error when it can't find them, e.g.:

  xkbcommon: ERROR: ~/.XCompose:4:9: failed to expand %L to the locale Compose file
  xkbcommon: ERROR: ~/.XCompose:4:12: unterminated string literal

Signed-off-by: Bryce Harrington <bryce@osg.samsung.com>
Signed-off-by: Ran Benita <ran234@gmail.com>