Add initial .gitignore file.

We ignore all generated files, apart from the *.c files which are needed
to be able to bootstrap nuweb.

.gitignore

@@ -0,0 +1,11 @@
+# .gitignore for nuweb
+*.o
+*.swp
+nuweb
+nuweb.tex
+nuweb.1
+*.aux
+*.dvi
+*.log
+*.out
+*.toc