Update README.md (#487)

Update README with new build instructions since we are no longer using the ios and android folders. So, ideally, devs will never need to open XCode and Android Studio (thank the gods)

README.md

@@ -8,7 +8,7 @@
   - brew tap wix/brew
   - brew install applesimutils
 - After initial setup:
-  - `cd ios ; pod install`
+  - `npx expo prebuild` -> you will also need to run this anytime `app.json` or `package.json` changes
 - Start the dev servers
   - `git clone git@github.com:bluesky-social/atproto.git`
   - `cd atproto`
@@ -26,8 +26,6 @@
   - Each test run: `yarn e2e:run`
 - Tips
   - `npx react-native info` Checks what has been installed.
-  - On M1 macs, [you need to exclude "arm64" from the target architectures](https://stackoverflow.com/a/65399525)
-    - Annoyingly this must be re-set via XCode after every pod install
   - The android simulator won't be able to access localhost services unless you run `adb reverse tcp:{PORT} tcp:{PORT}`
     - For instance, the localhosted dev-wallet will need `adb reverse tcp:3001 tcp:3001`
   - For some reason, the typescript compiler chokes on platform-specific files (e.g. `foo.native.ts`) but only when compiling for Web thus far. Therefore we always have one version of the file which doesn't use a platform specifier, and that should bee the Web version. ([More info](https://stackoverflow.com/questions/44001050/platform-specific-import-component-in-react-native-with-typescript).)