add cache.shared config reference and use case

public/static/docs/command-reference/config.md

@@ -152,6 +152,13 @@ for more details.) This section contains the following options:
   > These warnings are automatically turned off when `cache.type` is manually
   > set.
 
+- `cache.shared` - permissions for newly created or downloaded cache files and
+  directories. The default permissions are `0o664`(rw-r--r--) for files and
+  `0o755`(rwxr-xr-x) for directories. The only accepted value right now is
+  `group`, which makes DVC use `0o664`(rw-rw-r--) for files and
+  `0o775`(rwxrwxr-x) for directories, which is useful when you are using a a
+  [shared development server](/doc/use-cases/shared-development-server).
+
 - `cache.local` - name of a local remote to use as cache directory. (Refer to
   `dvc remote` for more information on "local remotes".) This will overwrite the
   value provided to `dvc config cache.dir` or `dvc cache dir`.

public/static/docs/use-cases/shared-development-server.md

@@ -37,6 +37,16 @@ to simply move it from an old cache location to the new one:
 $ mv .dvc/cache/* /path/to/dvc-cache
 ```
 
+Now you need to ensure that cache files/directories have appropriate
+permissions, so that they could be accessed by your collegues that are members
+of the same group:
+
+```dvc
+$ sudo find /path/to/dvc-cache -type f -exec chmod 0664 {} \;
+$ sudo find /path/to/dvc-cache -type d -exec chmod 0775 {} \;
+$ sudo chown -R myuser:ourgroup /path/to/dvc-cache/
+```
+
 ## Configure shared cache
 
 Tell DVC to use the directory we've set up above as an shared cache location by
@@ -46,6 +56,13 @@ running:
 $ dvc config cache.dir /path/to/dvc-cache
 ```
 
+And tell DVC to set group permissions on the newly created/downloaded cache
+files:
+
+```dvc
+$ dvc config cache.shared group
+```
+
 Commit changes to `.dvc/config` and push them to your git remote:
 
 ```dvc