Added some documentation to help newcomers send Keysend payments

[brianoflondon]

Change Description

It's documentation. It might nelp.

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[lightninglabs-deploy]

@brianoflondon, remember to re-request review from reviewers when ready

[brianoflondon]

@brianoflondon, remember to re-request review from reviewers when ready

I don't really have time to spend. This is a useful code example that would help newbies.

Publish or delete, lightning needs more and better documentation in my opinion but as you don't pay i have to spend more time on the project that does pay me.

[guggero]

Fair enough, thanks for the response and the initial contribution! We'll see if someone from the team has time to polish the text so we can merge it to our docs.

[guggero]

Added to our builder's guide here: https://docs.lightning.engineering/lightning-network-tools/lnd/send-messages-with-keysend#docs-internal-guid-5736b677-7fff-7369-1bb3-cf686146b756
Thanks for the initial text, @brianoflondon.