update the installation instructions

Building into a subdirectory prevents the build process from polluting
the source tree, simplifying the gitignore temendously.

.gitignore

@@ -1,34 +1,4 @@
-*.o
-*.log
-*.trs
 *.sw?
 *~
 
-.deps
-.dirstamp
-Makefile
-Makefile.in
-
-/aclocal.m4
-/autom4te.cache
-/compile
-/configure
-/depcomp
-/install-sh
-/missing
-/stamp-h1
-/test-driver
-
-/docs/Doxyfile
-/docs/html
-/docs/latex
-/docs/oshu.1
-/docs/oshu-library.1
-
-/share/samples*
-
-/src/liboshu.a
-/src/oshu
-/src/oshu-library
-/src/tests/zerotokei
-/oshu-*.tar.*
+/build/

README.md

@@ -69,23 +69,24 @@ First of all, make sure you have the required dependencies. These things are so
 common that I may assure you they're in your distribution's official
 repositories.
 
+- CMake 3.10 or newer,
+- a C++14 compiler,
+- pkg-config,
 - SDL2 and SDL2_image,
 - ffmpeg or libav,
 - cairo,
-- pango,
-- a C++14 compiler,
-- pkg-config,
-- autoconf, automake (when building from the git repository).
+- pango.
 
-oshu! is a standard autoconf'd project, so install it the regular way with:
+To build oshu!, follow the standard CMake procedure:
 
-	# ./autogen.sh if the configure file is missing.
-	./configure --prefix="$HOME/.local"
+	mkdir build
+	cd build
+	cmake -DCMAKE_INSTALL_PREFIX=~/.local ..
 	make && make install
 
 If you do use the `$HOME/.local` prefix, make sure you add `~/.local/bin` to
-your PATH, or invoke oshu! by specifying the full path like `~/.local/bin/oshu
-BEATMAP.osu`. Otherwise you may use the default `/usr/local`.
+your PATH, or invoke oshu! by specifying the full path like
+`~/.local/bin/oshu BEATMAP.osu`.
 
 If you want to have oshu integrated in your desktop: