Add CHANGELOG

[mptre]

The diff between the previous release v1.4.0 and master is quite
massive. In order to make the release process less tedious I propose
keeping a changelog up-to-date during development. The changelog can
then used as a boilerplate for the release notes.

We're missing out on some context related to the changes by not
referencing the corresponding PR. When adding future entries to the
changelog, the PR should be mentioned.

CHANGELOG.md was generated using the following script, with some
Vim post-processing:

body() {
    awk '
    /markdown-body/ { body = 1; next }
    body == 1 && /<\/div/ { exit 0 }
    body == 1 && length > 0 { sub(/^ +/, ""); print }
    '
}

htmltomd() {
    local tag=$1 sub=$2

    sed "s|<${tag}>\([^<]*\)</${tag}>|${sub}|g"
}

exec 1>CHANGELOG.md

git tag -l --format='%(contents:subject) %(taggerdate:short)' | sort -r | while read tag date; do
    printf '# %s - %s\n\n' "$tag" "$date"

    ftp -o - "https://github.com/thoughtbot/pick/releases/tag/${tag}" \
        | body \
        | sed 's|</*ul>||' \
        | htmltomd h1 '## \1' \
        | htmltomd code '`\1`' \
        | htmltomd li '- \1'
done

[calleluks]

Looks great, @mptre! Thanks for the great initiative.