maint: set next release number to 3.1.

* configure.ac (AC_INIT): Set next release number to 3.1.

Signed-off-by: Gary V. Vaughan <gary@gnu.org>
maint: update README.

* README: Move from here...
* README.md: ...to here.
Rewrite in standard markdown, and fix out-of-date text.
* .gitignore: Add README.

Signed-off-by: Gary V. Vaughan <gary@gnu.org>
configury: AC_CONFIG_MACRO_DIRS is not available in older autotools.

* configure.ac (AC_CONFIG_MACRO_DIRS): Rename from this...
(AC_CONFIG_MACRO_DIR): ...to this.

Signed-off-by: Gary V. Vaughan <gary@gnu.org>