DOC: Explanation for git submodule

docs/dev/intro.md

@@ -31,6 +31,12 @@ most cases we typically want to test for. The `sample-files` might cover a lot
 more edge cases, the behavior we get when file sizes get bigger, different
 PDF producers.
 
+In order to get the sample-files folder, you need to execute:
+
+```
+git submodule update --init
+```
+
 ## Tools: git and pre-commit
 
 Git is a command line application for version control. If you don't know it,
@@ -67,6 +73,8 @@ The `PREFIX` can be:
 * `ENH`: A new feature! Describe in the body what it can be used for.
 * `DEP`: A deprecation - either marking something as "this is going to be removed"
    or actually removing it.
+* `PI`: A performance improvement. This could also be a reduction in the
+        file size of PDF files generated by PyPDF2.
 * `ROB`: A robustness change. Dealing better with broken PDF files.
 * `DOC`: A documentation change.
 * `TST`: Adding / adjusting tests.