Update filesystem readme.md to include exists method
minor #12468 Update filesystem readme.md to include exists method (ericduran)

This PR was submitted for the master branch but it was merged into the 2.3 branch instead (closes #12468).

Discussion
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Update filesystem readme.md to include exists method

Read the contributing docs, I don't think I missed anything but to be fair I went through it pretty fast :-/

Commits
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16c1b66 Update filesystem readme.md to include exists method