Add UPDATING guide on how to update the example

This example has a few dependencies. Explain how to update each of them.

Author: Patater

UPDATING.md

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+## Updating this example's dependencies
+
+When you feel like modifying any of this example's dependencies (Mbed Crypto,
+Cryptoauthlib), for example to update the secure element driver interface,
+you'll need to make your changes across a handful of different components.
+
+1. Update SE driver interface (and tests) in Mbed Crypto (`mbed-crypto`)
+1. Save the changes to a branch in your fork of Mbed Crypto (`mbed-crypto`)
+1. Update Mbed OS (`mbed-os`) with your Mbed Crypto fork's SE driver changes
+1. Save the changes to a branch in your fork of Mbed OS (`mbed-os`)
+    1. `cd features/mbedtls/mbed-crypto/importer`
+    1. Within the `Makefile`, edit `CRYPTO_RELEASE` to point to your branch
+    1. Within the `Makefile`, edit `CRYPTO_REPO_URL` to point to your fork
+    1. `make update`; if you have an `mbed-crypto` in a different repo already
+       checked out, this command can fail. Use `rm -rf
+       TARGET_IGNORE/mbed-crypto/` and run `make update` again
+    1. `make all`
+    1. Commit your changes on a branch in your fork
+1. Update the ATECC608A driver for Mbed OS (`mbed-os-atecc608a`) for the
+   interface changes
+1. Save the changes to a branch in your fork of `mbed-os-atecc608a`
+1. Update the Mbed OS example for ATECC608A (`mbed-os-example-atecc608a`)
+    1. For each example subfolder you want to update:
+        1. Point the `mbed-os.lib` file at the branch in your fork
+        1. Point the `mbed-os-atecc608a.lib` file at the branch in your fork
+        1. `mbed deploy`