Add CONTRIBUTING.md to help new contributors with UI testing issues

CONTRIBUTING.md

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+# Welcome to ttyplot!
+
+This file documents how to contribute to ttyplot and
+things to know before facing them.  It will likely
+never be complete, but if something important
+turns out to be missing here, let's add it!
+
+
+## Problem: The CI complains that two images are too dissimilar
+
+The CI does basic UI testing by making screenshots
+of the running application and comparing them
+against pre-recorded screenshots `recordings/expected-*.png`.
+When you make changes to ttyplot that change the *runtime appearance* of
+ttyplot, the CI will hopefully catch these changes
+and reject them as a regression.  In a case where these
+changes are made with full intention, the images
+that the CI is comparing to at `recordings/expected-*.png`
+will then need to be regenerated.
+For your convenience, an easy way to do that is:
+
+```console
+$ ./recordings/get_back_in_sync.sh
+```
+
+The script will re-render these images and even create a Git commit
+for you.
+
+For all that to work well on macOS, you would need to install:
+
+```console
+$ brew tap homebrew/cask-fonts
+$ brew install \
+    agg \
+    asciinema \
+    coreutils \
+    font-liberation
+```
+
+On a Debian-based Linux including Ubuntu, you would need to install…
+
+```console
+$ sudo apt-get update
+$ sudo apt-get install --no-install-recommends -V \
+    asciinema \
+    ca-certificates \
+    cargo \
+    fonts-liberation
+$ cargo install --git https://github.com/asciinema/agg
+```
+
+…and put the install location of `agg` into `${PATH}`:
+
+```console
+$ export PATH="${PATH}:${HOME}/.cargo/bin"
+```