quick doc on flavors and migration

docs/rust_crypto_integration.md

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+## Overview
+
+Until the final migration to [rust crypto sdk](https://github.com/matrix-org/matrix-rust-components-kotlin), the Element Android project will support two
+different SDK as a product flavor.
+
+The `matrix-sdk-android` module is defining a new flavor dimension `crypto`, with two flavors `kotlinCrypto` and `rustCrypto`.
+The crypto module cannot be changed at runtime, it's a build time configuration. The app supports migration from kotlinCrypto to rustCrypto but not the other
+way around.
+
+The code that is not shared between the flavors is located in dedicated source sets (`src/kotlinCrypto/`, `src/rustCrypto/`). Some tests are also extracted
+in different source sets because they were accessing internal API and won't work with the rust crypto sdk.
+
+## Noticeable changes
+
+As a general rule, if you stick to the `kotlinCrypto` the app should behave as it was before the integration of favours.
+There is a noticeable exception though:
+In order to integrate the rust crypto several APIs had to be migrated from callback code to suspendable code. This change
+impacted a lot the key verification engine (user and device verification), so this part has been refactored for `kotlinCrypto`. The UI is also impacted,
+the verification flows now match the web experience.
+
+TLDR; Verification UI and engine has been refactored.
+
+## Testing with a local rust aar
+
+In order to run a custom rust SDK branch you can follow the direction in the [bindings repository](https://github.com/matrix-org/matrix-rust-components-kotlin) 
+in order to build the `matrix-rust-sdk-crypto.aar`.
+
+Copy this lib in `library/rustCrypto/`.
+
+Then go to `matrix-sdk-android/build.gradle` and toggle the comments between the following lines.
+
+````
+// rustCryptoImplementation files('libs/matrix-rust-sdk-crypto.aar')
+rustCryptoImplementation("org.matrix.rustcomponents:crypto-android:0.3.4")
+````
+
+## Changes in CI
+
+The workflow files have been updated to use the `kotlinCrypto` flavor, e.g
+`assembleGplayNightly` => `assembleGplayKotlinCryptoNightly`
+
+An additional workflow has been added to build the `rustCrypto` flavor (elementr.yml, ` Build debug APKs ER`).
+
+
+## Database migration from kotlin to rust
+
+With the kotlin flavor, the crypto information are persisted in the crypto realm database.
+With the rust flavor, the crypto information are in a sqllite database.
+
+The migration is handled when injecting `@SessionRustFilesDirectory` in the olmMachine. 
+When launching the first time after migration, the app will detect that there is no rust data repository and it will
+create one. If there is an existing realm database, the data will then migrated to rust. See `ExtractMigrationDataUseCase`.
+This will extract your device keys, account secrets, active olm and megolm sessions.
+
+There is no inverse migration for now, as there is not yet rust pickle to olm pickle support in the sdk.
+
+If you migrate your app to rust, and want to revert to kotlin you have to logout then login again.
+