contributing/index.md: Add port requirement for PopCorn

Author: cinerea0

src/contributing/index.md

@@ -20,6 +20,7 @@ If you would like to contribute usage reports, the
 [PopCorn](https://github.com/the-maldridge/popcorn) program reports installation
 statistics back to the Void project. These statistics are purely opt-in -
 PopCorn is *not* installed or enabled by default on any Void systems.
+Additionally, PopCorn requires that port 8001 not be blocked on your system.
 
 *PopCorn* only reports which packages are installed, their version, and the host
 CPU architecture (the output of `xuname`). This does not report which services