Explain how and why we use bitcoin core

[sstone]

Explain why we chose to delegate most onchain tasks to bitcoin core (including onchain wallet management), the additional requirements that it creates and also the benefits in terms of security.

[t-bast]

A few nits, otherwise LGTM

[codecov-commenter]

Codecov Report

Merging #2473 (9a21baa) into master (3b12475) will increase coverage by 0.06%.
The diff coverage is 93.98%.

@@            Coverage Diff             @@
##           master    #2473      +/-   ##
==========================================
+ Coverage   84.92%   84.99%   +0.06%     
==========================================
  Files         198      198              
  Lines       15783    15812      +29     
  Branches      637      685      +48     
==========================================
+ Hits        13404    13439      +35     
+ Misses       2379     2373       -6     

Impacted Files	Coverage Δ
...in/scala/fr/acinq/eclair/channel/ChannelData.scala	98.11% <ø> (ø)
.../main/scala/fr/acinq/eclair/db/DualDatabases.scala	11.27% <0.00%> (+0.32%)	⬆️
.../src/main/scala/fr/acinq/eclair/db/NetworkDb.scala	100.00% <ø> (ø)
.../eclair/wire/protocol/LightningMessageCodecs.scala	100.00% <ø> (ø)
...in/scala/fr/acinq/eclair/channel/fsm/Channel.scala	86.27% <66.66%> (+0.11%)	⬆️
...main/scala/fr/acinq/eclair/router/Validation.scala	95.65% <87.23%> (+0.22%)	⬆️
.../scala/fr/acinq/eclair/message/OnionMessages.scala	81.81% <90.00%> (+5.45%)	⬆️
...src/main/scala/fr/acinq/eclair/router/Router.scala	94.20% <93.33%> (+0.02%)	⬆️
...r-core/src/main/scala/fr/acinq/eclair/Eclair.scala	52.10% <100.00%> (+1.56%)	⬆️
...re/src/main/scala/fr/acinq/eclair/NodeParams.scala	93.28% <100.00%> (+0.02%)	⬆️
... and 21 more

[remyers]

LGTM!

It might be useful to collaborate on a best-practices document to explore the interface between Lightning and on-chain where we could get into more technical details about the trade-offs.

[remyers]

Looks good!