Exclude some files from release tar balls

Git-specific files don't make sense outside of Git repository and CI
files are not needed neither.

.gitattributes

@@ -36,3 +36,11 @@ configure.in eol=lf
 
 # Ignore changes in the generated files.
 build/upmake    -diff
+
+# Don't include stuff which is only used with Git in the archives.
+.github/                        export-ignore
+.gitattributes                  export-ignore
+.gitignore                      export-ignore
+.gitmodules                     export-ignore
+.travis.yml                     export-ignore
+appveyor.yml                    export-ignore