docs: add instructions how to sign a LND release.

Author: ziggie1984

docs/signing_release_candidate.md

@@ -0,0 +1,70 @@
+# Signing a LND Release
+
+When a new version of LND is released, binaries for the lnd and lncli programs
+are provided for various platforms and CPU architectures. The hashes of all
+these binaries are written into a file known as the "Manifest". This Manifest is
+then signed by a quorum of trusted LND contributors (see [verify-install.sh](/scripts/verify-install.sh)
+script for more details), ensuring that users can trust the binaries they 
+download, knowing they haven't been modified during the automated build process.
+
+To verify a release binary, users have two options:
+
+* Manual Verification: Users can manually download the signature files and 
+Manifest from GitHub LND release page, then verify the PGP signatures and 
+hashes.
+
+* Automated Verification: The LND repository provides a script, 
+[verify-install.sh](/scripts/verify-install.sh), that automates the verification process. This script uses a
+set of trusted developer keys (located in the repo under [scripts/keys/](/scripts/keys)) and
+downloads the necessary data from the GitHub server to verify the integrity of
+the local lnd/lncli binaries.
+
+Running [verify-install.sh](/scripts/verify-install.sh) validates that trusted developers attest to the authenticity between the lnd release binaries hosted on Github and the developer's local builds.
+
+## Adding a new developer as a signer
+
+When another developer is added to the trusted group of people which are
+allowed to sign the lnd/lncli releases, their public PGP key needs to be added to
+the LND repo. These keys are added in a PR in which 2 reviewers ensure the developer
+is in possession of the PGP key which will be added to the LND repo.
+(See https://github.com/lightningnetwork/lnd/pull/8788 as an example).
+It is important that the name of the PGP key equals the name in the
+[verify-install.sh](/scripts/verify-install.sh) script. See also [scripts/keys/README.md](/scripts/keys/README.md) for more information.
+
+## Signing a release binary package
+
+If the new developer's PGP key has been successfully added to the LND repository,
+through the aforementioned PR example, they are now able to provide their
+signature for the new release's "Manifest" file. To do so, the developer must
+follow these steps:
+
+* Follow the build instructions at https://github.com/lightningnetwork/lnd/blob/masterdocs/release.md#building-a-new-release.
+
+* After a successful build, all binaries and Manifest files, will be placed
+in a directory, named after the tag, created within the directory in which the build occurred. For 
+instance, in the case mentioned above, the folder will be named 
+`lnd-v0.18.3-beta`.
+Ensure that the SHA-256 hashes, in your locally-generated Manifest file, match
+those in the Manifest file of the official release on the LND GitHub repository.  
+Tip: Download the official release Manifest file to your local maschine and do:  
+`diff lnd-v0.18.3-beta/manifest-v0.18.3-beta.txt ~/Downloads/manifest-v0.18.3-beta.txt`
+(example command for a release candidate called `v0.18.3-beta`)  
+Only if all hashes are identical, should you sign the release.  If the digests
+match, see the example signing comand, assuming your PGP signing key is
+available on your local device:  
+`gpg --local-user $KEYID  --detach-sig --output manifest-$USERNAME-v0.18.3-beta.sig manifest-v0.18.3-beta.txt`.  
+`USERNAME` being the name in the `[verify-install.sh](../scripts/verify-install.sh)`
+script and also the name of your PGP key file. The whole argument `--local-user $KEYID`
+is only needed if there's more than one signing key on your local machine. Be
+sure to substitute the TAG value `v0.18.3-beta` with the version you are 
+currently signing.
+
+* Finally, upload the signature file
+(e.g. manifest-USERNAME-v0.18.3-beta.sig) to the GitHub release page.
+Github write permissions are required to upload signatures to the LND release
+page. To avoid interfering with other signers who may be updating the GitHub
+release page, LND developers use a `KeyBase` communication channel to signal
+when an edit is in progress. Once your signature file is successfully uploaded
+and the release page is unlocked, the signing process is complete.
+
+Congratulations signing the LND release 🎉.