Add some exceptions in .gitignore.

A user reports that our top-level .gitignore ignores several files
that are actually part of the real git repository. This is
inconvenient if you start from a downloaded tarball or zip file, and
try to make it _back_ into a git repository to work with it.

The blanket rule to ignore files called "Makefile" (on the theory that
they're autogenerated by cmake, or in the pre-cmake days, by
autotools) was also excluding two handwritten Makefiles, in 'icons'
and in 'contrib/cygtermd'. And the rule about doc/*.txt, intended to
exclude Halibut's plain-text output, also excluded doc/CMakeLists.txt.

With these exclusions in place, if you download a PuTTY source
.tar.gz, unpack it, change into the unpacked subdirectory, and run
'git init', 'git add .' and 'git commit', then 'git status --ignored'
to see what files in the tarball weren't added to the repo, you'll
find that the remaining ones are all in the 'doc' directory, and
really _are_ Halibut outputs: all the man pages (putty.1 etc), the
Windows help file putty.chm, and the plain text puttydoc.txt.

.gitignore

@@ -72,12 +72,15 @@ cmake_install.cmake
 /build.out
 /empty.h
 Makefile
+!/contrib/cygtermd/Makefile
+!/icons/Makefile
 /compile
 *.a
 /charset/sbcsdat.c
 /contrib/cygtermd/cygtermd.exe
 /doc/*.html
 /doc/*.txt
+!/doc/CMakeLists.txt
 /doc/*.cnt
 /doc/*.hlp
 /doc/*.gid