Added note for configuration refactor

docs/docs/upgrading.md

@@ -11,8 +11,39 @@ description: >-
 
 This page contains the list of deprecations and important or breaking changes for pomerium `v0.0.4` compared to `v0.0.5`. Please read it carefully.
 
-### X
+### Breaking: POLICY_FILE removed
 
-### Y
+Usage of the POLICY_FILE envvar is no longer supported.  Support for file based policy configuratin has been shifted into the new unified config file.  
+
+### Important: Configuration file support added
+
+* Pomerium now supports an optional -config flag.  This flag specifies a file from which to read all configuration options.  It supports yaml, json, toml and properties formats.
+
+* All options which can be specified via MY_SETTING style envvars can now be specified within your configuration file as key/value.  The key is generally the same as the envvar name, but lower cased.  See Reference Documentation for exact names.
+
+* Options precedence is ENV > Config > Defaults
+
+* The options file supports a policy key, which contains policy in the same format as POLICY_FILE.  To convert an existing policy.yaml into a config.yaml, just move your policy under a policy key.
+
+  Old:
+  ```yaml
+  - from: httpbin.corp.beyondperimeter.com
+    to: http://httpbin
+    allowed_domains:
+      - pomerium.io
+    cors_allow_preflight: true
+    timeout: 30s
+  ```
+
+  New:
+  ```yaml
+  policy:
+    - from: httpbin.corp.beyondperimeter.com
+      to: http://httpbin
+      allowed_domains:
+        - pomerium.io
+      cors_allow_preflight: true
+      timeout: 30s
+  ```
 
 ### Z