Document how mat2 compares to other software

doc/comparison_to_others.md

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+# Exiftool
+
+mat2 is in fact using exiftool to extract metadata from files,
+but not to remove them. The previous iteration of mat2, MAT,
+was using exiftool to remove metadata, which lead to several cases where
+they weren't correctly removed, if at all.
+For example, [Exiftool's documentation](https://www.sno.phy.queensu.ca/~phil/exiftool/TagNames/PDF.html)
+states the following with regard to PDF:
+
+> All metadata edits are reversible. While this would normally be considered an
+advantage, it is a potential security problem because old information is never
+actually deleted from the file.
+
+To remove metadata, mat2 usually re-render the file completely, eliminating
+all possible original metadata. See the `implementation_notes.md` file for
+details.
+
+
+# jpegoptim, optipng, …
+
+While designed to reduce as much as possible the size of pictures,
+those software can be used to remove metadata. They usually have very good
+support for a single picture format, and can be used in place of mat2 for them.
+
+
+# PDF Redact Tools
+
+[PDF Redact Tools](https://github.com/firstlookmedia/pdf-redact-tools) is
+a software developed by the people from [First Look
+Media](https://firstlook.media/), the entity behind, amongst other things, 
+[The Intercept](https://theintercept.com/).
+
+The tool uses roughly the same approach than mat2 to deal with PDF,
+which is unfortunately the only fileformat that it does support.
+It's interesting to note that it has counter-measures against
+[yellow dots](https://en.wikipedia.org/wiki/Machine_Identification_Code),
+a capacity that mat2 [doesn't possess yet](https://0xacab.org/jvoisin/mat2/issues/43).
+
+
+# Exiv2
+
+[Exiv2](https://www.exiv2.org/) was considered for mat2,
+but it currently [misses a lot of metadata](https://0xacab.org/jvoisin/mat2/issues/85)
+
+
+# Others non open source software/online service
+
+There are a lot of closed-source software and online service claiming to remove
+metadata from your files, but since there is no way to actually verify that
+they're effectively removing them, let alone adding unique markers, they
+shouldn't be used.