add policy and docs about api-breaking changes

Author: inconshreveable

README.md

@@ -49,14 +49,22 @@ func doUpdate(url string) error {
 - Release tooling with proper code signing
 - Update/download metrics
 
-## Breaking API Changes
-- Sept 3, 2015: The `Options` struct passed to `Apply` was changed to be passed by value instead of passed by pointer.
+## API Compatibility Promises
+The master branch of `go-update` is *not* guaranteed to have a stable API over time. For any production application, you should vendor
+your dependency on `go-update` with a tool like git submodules, [gb](http://getgb.io/) or [govendor](https://github.com/kardianos/govendor).
+
+The `go-update` package makes the following promises about API compatibility:
+1. A list of all API-breaking changes will be documented in this README.
+1. `go-update` will strive for as few API-breaking changes as possible.
+
+## API Breaking Changes
+- Sept 3, 2015: The `Options` struct passed to `Apply` was changed to be passed by value instead of passed by pointer. Old API at `28de026`.
+- Aug 9, 2015: 2.0 API. Old API at `221d034` or `gopkg.in/inconshreveable/go-update.v0`.
 
 ## Older API Versions
 Did your build just break because the go-update API changed? You have two options:
 
 1. Update your import to `gopkg.in/inconshreveable/go-update.v0`
-1. Vendor your dependency on it with a tool like [gb](http://getgb.io/) or [govendor](https://github.com/kardianos/govendor)
 
 ## License
 Apache