Add exiftool to prerequisites

I got an internal server error upon uploading a sign in/sign up page logo in Admin > Appearance:

12:28:00 gitlab-workhorse.1      | time="2019-04-17T12:28:00-07:00" level=info msg="running exiftool to remove any metadata" correlation_id=GZFyJPNSMIa filename=rails_sample.jpg
12:28:00 gitlab-workhorse.1      | time="2019-04-17T12:28:00-07:00" level=error msg=error correlation_id=GZFyJPNSMIa error="handleFileUploads: extract files from multipart: failed to start EXIF metadata cleaner: start [exiftool -all= --IPTC:all --XMP-iptcExt:all -tagsFromFile @ -ResolutionUnit -XResolution -YResolution -YCbCrSubSampling -YCbCrPositioning -BitsPerSample -ImageHeight -ImageWidth -ImageSize -Copyright -CopyrightNotice -Orientation -]: exec: \"exiftool\": executable file not found in $PATH" method=POST uri=/admin/appearance
12:28:00 gitlab-workhorse.1      | localhost:3001 127.0.0.1:64791 - - [2019/04/17:12:28:00 -0700] "POST /admin/appearance HTTP/1.1" 500 22 "http://localhost:3001/admin/appearance" "Mozilla/5.0 (Macintosh; Intel Mac OS X 10.14; rv:66.0) Gecko/20100101 Firefox/66.0" 0.019
12:28:00 gitlab-workhorse.1      | time="2019-04-17T12:28:00-07:00" level=error msg=error correlation_id=JhVStbioIl6 error="badgateway: failed after 0s: context canceled" method=GET uri=/favicon.ico

Uploading a logo succeeded after:

brew install exiftool

Is it correct to just add exiftool to prerequisite docs?

cc @jprovaznik