Add install of 'p7zip-full' to Travis configuration and docs

travis/.travis.yml.example-full

@@ -31,10 +31,14 @@ addons:
     - g++-mingw-w64-x86-64
     # for RTEMS cross builds
     - qemu-system-x86
+    # for .zip/.7z archive hooks
+    - p7zip-full
   homebrew:
     packages:
     # for the sequencer
     - re2c
+    # for .zip/.7z archive hooks
+    - p7zip
     update: true
 
 install:

travis/README.md

@@ -60,3 +60,9 @@ configuration.)
 
 Caches are automatically removed after approx. four weeks.
 Your jobs will have to rebuild them once in a while.
+
+## Miscellanea
+
+To use the feature to extract `.zip`/`.7z` archives by setting
+`*_HOOK` variables, the Linux and MacOS runners need the APT package
+`p7zip-full` resp. the Homebrew package `p7zip` installed.