Move docker-compose.env to docker-compose.env.example & adjust docs

This file, like `docker-compose.yml`, should be edited by the user. To
avoid merge conflicts when pulling updates, the edited version should
not be committed to the repository.

docker-compose.env.example

@@ -0,0 +1,15 @@
+# Environment variables to set for Paperless
+# Commented out variables will be replaced by a default within Paperless.
+
+# Passphrase Paperless uses to encrypt and decrypt your documents
+PAPERLESS_PASSPHRASE=CHANGE_ME
+
+# The amount of threads to use for text recognition
+# PAPERLESS_OCR_THREADS=4
+
+# Additional languages to install for text recognition
+# PAPERLESS_OCR_LANGUAGES=deu ita
+
+# You can change the default user and group id to a custom one
+# USERMAP_UID=1000
+# USERMAP_GID=1000