Document how to prepare video clips from user testing recordings

wiki/src/contribute/how/user_experience.mdwn

@@ -29,6 +29,7 @@ Resources
   - [[Survey platform (*LimeSurvey*)|user_experience/limesurvey]]
   - [[Guidelines for user testing|user_experience/testing]]
   - [[Recording user testing|user_experience/recording]]
+  - [[Preparing video clips|user_experience/clip]]
   - Participant Bill of Rights (adapted from
     [Simply Secure](https://simplysecure.org/knowledge-base/)):
     - [English ODT](https://github.com/sajolida/tails-ux/raw/master/tools/bill_of_rights.en.fodt)

wiki/src/contribute/how/user_experience/clip.mdwn

+[[!meta title="Preparing video clips"]]
+
+Cut out a video clip from a longer recording
+--------------------------------------------
+
+    ffmpeg -ss [start] -t [duration] -i [input] [output]
+
+For example:
+
+    ffmpeg -ss 25:20 -t 10 -an -i P4.ogv clip.webm
+
+Amplify the audio in a video clip
+---------------------------------
+
+    ffmpeg -i [input] -af "volume=25dB" [output]
+
+For example:
+
+    ffmpeg -i clip.webm -af "volume=25dB" clip-amplified.webm
+
+Remove the sound in a video clip
+--------------------------------
+
+    ffmpeg -i [input] -an [output]
+
+For example:
+
+    ffmpeg clip.webm -an clip-nosound.webm