Submit 'Old Flags to Platforms Migration' for review

Design proposal for bazelbuild/bazel#6426

Author: aragos

README.md

@@ -53,6 +53,7 @@ list.
 
 | Last updated | Title                                                                                                                                | Author(s) alias                                                              | Category              |
 | ------------ | ------------------------------------------------------------------------------------------------------- | ------------------------------------------ | -------- |
+|   2018-10-17 | [Old Flags to Platform Migration](https://docs.google.com/document/d/1Vg_tPgiZbSrvXcJ403vZVAGlsWhH9BUDrAxMOYnO0Ls) | [@aragos](https://github.com/aragos) | Configurability |
 |   2018-10-10 | [Platform Inheritance](designs/2018-10-09-platform-inheritance.md)                                                              | [@katre](https://github.com/katre)       | Configurability |
 |   2018-08-07 | [Repository Remapping Revisited](https://docs.google.com/document/d/1YBltt_jd4QTRzxkp1amsgfS6P8yBVDF4n1KGb3jcf_w/edit?usp=sharing)                                                                   | [@dkelmer](https://github.com/dkelmer)                                           | External Repositories |
 |   2018-07-04 | [Update Download Functions in repository_ctx](https://docs.google.com/document/d/1znnk1u3kdl4iOh3ptmJnW7nHr8UtkVl__Au4atLzNKc)       | [@dkelmer](https://github.com/dkelmer)                                       | External Repositories |