add docs about minimal deletion

Author: AbhiPrasad

MIGRATION.md

@@ -10,6 +10,28 @@ Node.js version 6 has reached end of life in April 2019. For Sentry JavaScript S
 As far as SDK development goes, dropping support means no longer running integration tests for Node.js version 6, and also no longer handling edge cases specific to version 6.
 Running the new SDK version on Node.js v6 is therefore highly discouraged.
 
+## Removal of `@sentry/minimal`
+
+The `@sentry/minimal` package was deleted and it's functionality was moved to `@sentry/hub`. All exports from `@sentry/minimal` should be avaliable in `@sentry/hub` other than `_callOnClient` function which was removed.
+
+```ts
+// New in v7:
+import {
+  addBreadcrumb,
+  captureException,
+  configureScope,
+  setTag,
+} from '@sentry/hub';
+
+// Before:
+import {
+  addBreadcrumb,
+  captureException,
+  configureScope,
+  setTag,
+} from '@sentry/minimal';
+```
+
 ## Removal Of Old Platform Integrations From `@sentry/integrations` Package
 
 The following classes will be removed from the `@sentry/integrations` package and can no longer be used:

packages/hub/src/exports.ts

@@ -15,6 +15,13 @@ import {
 import { getCurrentHub, Hub } from './hub';
 import { Scope } from './scope';
 
+// Note: All functions in this file are typed with a return value of `ReturnType<Hub[HUB_FUNCTION]>`,
+// where HUB_FUNCTION is some method on the Hub class.
+//
+// This is done to make sure the top level SDK methods stay in sync with the hub methods.
+// Although every method here has an explicit return type, some of them (that map to void returns) do not
+// contain `return` keywords. This is done to save on bundle size, as `return` is not minifiable.
+
 /**
  * Captures an exception event and sends it to Sentry.
  *