Remove broken link to INSTALL file from README

* README (Installation and Usage): Remove reference to INSTALL file
(not generated by autoreconf as AM_INIT_AUTOMAKE specifies "foreign"
strictness).
* .gitignore (INSTALL): Remove.

.gitignore

@@ -41,7 +41,6 @@ Makefile
 /pthreads-w32*
 
 # These files are generated by autoreconf:
-/INSTALL
 /aclocal.m4
 /compile
 /config.guess

README

@@ -31,8 +31,8 @@ Please address bug reports to gc@linux.hpl.hp.com
 
 The configuration and build scripts for this package were generated by
 Automake/Autoconf.  "./configure --prefix=<install dir>; make; make install"
-in this directory should work.  For a more customized build, see "INSTALL"
-and the output of "./configure --help".
+in this directory should work.  For a more customized build, see the output of
+"./configure --help".
 
 Note that much of the content of this library is in the header files.
 However, two small libraries are built and installed: