add instructions for CF

Author: ciur

docs/settings/main.md

@@ -21,6 +21,40 @@ Example:
       PAPERMERGE__MAIN__LOGGING_CFG=/etc/papermerge/logging.yaml
 
 
+## MAIN__FILE_SERVER
+
+Context: `webapp`.
+Who is serving document files? Is it `webapp` ? or CDN?
+This environment variable can have two values:
+
+* `local` (default)
+* `s3`
+
+With `local` (which is set by default), `webapp` is the one serving document files.
+With `s3` is CDN serving document files. At this point only AWS CloudFront is supported as CDN.
+If `PAPERMERGE__MAIN__FILE_SERVER=s3` then you need to provide following environment variables:
+
+* `PAPERMERGE__MAIN__CF_SIGN_URL_PRIVATE_KEY`
+* `PAPERMERGE__MAIN__CF_SIGN_URL_KEY_ID`
+* `PAPERMERGE__MAIN__CF_DOMAIN`
+
+
+## MAIN__CF_SIGN_URL_PRIVATE_KEY
+
+Used only in combination with `PAPERMERGE__MAIN__FILE_SERVER=s3`.
+`PAPERMERGE__MAIN__CF_SIGN_URL_PRIVATE_KEY` is the Cloud Front private key (as string).
+
+## MAIN__CF_SIGN_URL_KEY_ID
+
+Used only in combination with `PAPERMERGE__MAIN__FILE_SERVER=s3`.
+`MAIN__CF_SIGN_URL_KEY_ID` is the Cloud Front (CF) sign url key id.
+
+## MAIN__CF_DOMAIN
+
+Used only in combination with `PAPERMERGE__MAIN__FILE_SERVER=s3`.
+`MAIN__CF_DOMAIN` is the Cloud Front (CF) domain.
+
+
 ## MAIN__TIMEZONE
 
 Which timezone to use.

docs/setup/s3backend.md

@@ -139,4 +139,14 @@ $ pmcompose -u admin -p 1234 -lc -s3
 
 ## Content Delivery
 
-docs: work in progress
+To enable files delivery via CDN, you need to set, in `webapp`, `PAPERMERGE__MAIN__FILE_SERVER` to `s3`:
+
+`PAPERMERGE__MAIN__FILE_SERVER=s3`
+
+At this point the only CDN provider which can be used with {{extra.project}} is AWS Cloud Front.
+Besides `PAPERMERGE__MAIN__FILE_SERVER=s3`, you need to set, on `webapp`, following, cloud front specific,
+environment variables (CF=AWS Cloud Front):
+
+* `PAPERMERGE__MAIN__CF_SIGN_URL_PRIVATE_KEY`
+* `PAPERMERGE__MAIN__CF_SIGN_URL_KEY_ID`
+* `PAPERMERGE__MAIN__CF_DOMAIN`