minor #3570 Update .gitattributes to exclude non-dist files (kaznovac)

This PR was submitted for the 3.x branch but it was squashed and merged into the 1.x branch instead.

Discussion
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Update .gitattributes to exclude non-dist files

resolves #3503

some extra files can be excluded also:
- changelog
- license
- readme

should this be rebased to 1.x, 2.x?

Commits
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92766d0 Update .gitattributes to exclude non-dist files

.gitattributes

@@ -1,2 +1,3 @@
+/doc/** export-ignore
 /tests export-ignore
 /phpunit.xml.dist export-ignore