Add README with info on how to add iOS Password Managers.

ios/chrome/browser/ui/activity_services/README.md

Index: ios/chrome/browser/ui/activity_services/README.md
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+# iOS Application Specific Services in Chrome for iOS
+
+[UIActivity][1] is an abstract class for implementing app-specific services
+such as Share (to social media), Print, Reading List, and Password Management
+app extensions.
+
+## Adding iOS Password Managers App Extensions
+
+Chrome for iOS recognizes an action extension is a Password Manager app
+extension in one of the following two ways.
+
+1. By Bundle ID match. If the bundle ID for the app extension contains
+the substring `find-login-action`, it will be handled as a Password
+Manager app extension.
+
+1. By explicitly listing in [`activity_type_util.mm`][2]. The anonymous
+namespace function `IsPasswordManagerActivity()` in this file contains
+a static structure listing all the Password Manager app extensions
+known to Chrome for iOS. The first field is a string containing either
+the full bundle ID or the leading portion of the bundle ID. The second
+field is a flag to indicate whether a full bundle ID is expected or
+if the string is intended to be a prefix for matching bundle IDs.
+
+The first option is recommended because it does not require any code
+changes to Chrome. If an app extension meets the first condition, it
+works with current and previous versions of Chrome for iOS (since early
+2016). If for any reasons that an app extension cannot change its
+bundle ID, option 2 may be used. To add support to Chrome for iOS, submit
+a changelist similar to [this][3] for review.
+
+
+[1]: https://developer.apple.com/reference/uikit/uiactivity?language=objc
+[2]: ./activity_type_util.mm
+[3]: https://codereview.chromium.org/2820113002/