Ignore /notes in git-generated .zip archives

Inspired by @barryvdh's comment in #268. This dramatically decreases the
size of the generated archive (~50% savings).

```
$ git log --oneline -n 1
e29c7b6 bumped version to 5.3.2-DEV
$ git archive -o without_patch.zip HEAD .
$ echo "/notes export-ignore" >> ./.gitattributes
$ git ci -am "Ignore /notes when generating an archive"
$ git archive -o with_patch.zip HEAD .
$ du -h with{,out}_patch.zip
428K    with_patch.zip
1.0M    without_patch.zip
```