Added a template for .gitignore.

Contains the files and directories that are generated by running
./bootstrap.

It also masks out .gitignore and the recommended ./build directory.

To use run:
  cp .gitignore-template .gitignore

From: Charlie Carson <ccarson@twitter.com>
Review: https://reviews.apache.org/r/17001

.gitignore-template

@@ -0,0 +1,23 @@
+# First, ignore yourself.
+.gitignore
+
+# Files created by ./bootstrap.
+Makefile.in
+aclocal.m4
+autom4te.cache
+config.guess
+config.sub
+configure
+depcomp
+install-sh
+libtool.m4
+ltmain.sh
+ltoptions.m4
+ltsugar.m4
+ltversion.m4
+lt~obsolete.m4
+missing
+
+# Recommended build directory.
+/build/
+