[doc]add a doc about flashing

[romain1235]

Describe what should be documented
i would like a more detailed documentation about how to use flash_external and flash_internal
What type of doc should it be
maybe a doc in the readme and an example script.

[Yaya-Cout]

Hi, sorry I missed your issue

I think these methods just take a Blob or an ArrayBuffer. I wanted to send you a reference to Upsilon's website, but we directly use the lower-level method (https://github.com/UpsilonNumworks/Upsilon-website/blob/master/src/views/Installer.vue#L636).

Here's how Omega website uses it: https://github.com/Omega-Numworks/omega-numworks.github.io/blob/react-master/src/dfu/installer.ts#L286