Add towncrier (#2873)

* Adds towncrier as our changelog system.
* Updates our contributor guidelines for this.

Resolves #2872

tools/check_changelog_entries.sh

@@ -0,0 +1,19 @@
+#!/usr/bin/env bash
+
+# This can be run for other remotes than origin with the env vat `RED_REMOTE`
+REMOTE="${RED_REMOTE:-origin}"
+
+if [[ $(git diff "$REMOTE"/V3/develop) ]]; then
+
+  if [[ $(git diff "$REMOTE"/V3/develop -- changelog.d/) ]]; then
+    echo "Found changelog fragments..."
+    exit 0
+  else
+    echo "Error: No new changelog fragments!"
+    exit 1
+  fi
+
+else
+  echo "No changes to need changelog for."
+  exit 0
+fi

tools/primary_deps.ini

@@ -30,6 +30,7 @@ docs =
     Sphinx
     sphinx_rtd_theme
     sphinxcontrib-trio
+    towncrier
 mongo =
     dnspython
     motor