Document how to update the Nix flake derivation

Signed-off-by: Jack Baldry <jack.baldry@grafana.com>

RELEASES.md

@@ -0,0 +1,71 @@
+# Releases
+
+## Nix flake
+
+A [Nix Flake](https://nixos.wiki/wiki/Flakes) is provided that can be installed using the Nix package manager.
+On each release, the derivation `version` attribute needs to be updated with the release tag and the `vendorSha256` attribute with
+the checksum for the fixed output derivation for the vendored Go packages.
+
+> **Note:** The following steps require a 2.X release of the `nix` command.
+
+1. Update the `version` with the release tag.
+```patch
+--- a/nix/default.nix
++++ b/nix/default.nix
+@@ -3,7 +3,7 @@
+ with pkgs;
+ buildGoModule rec {
+   pname = "jsonnet-language-server";
+-  version = "0.6.3";
++  version = "<major>.<minor>.<patch>";
+
+   ldflags = ''
+     -X main.version=${version}
+```
+2. Replace the `vendorSha256` with `lib.fakeSha256`.
+```patch
+--- a/nix/default.nix
++++ b/nix/default.nix
+@@ -9,7 +9,7 @@ buildGoModule rec {
+     -X main.version=${version}
+   '';
+   src = lib.cleanSource ../.;
+-  vendorSha256 = "sha256-mGocX5z3wf9KRhE9leLNCzn8sVdjKJo6FzgP1OwQB3M=";
++  vendorSha256 = lib.fakeSha256;
+
+   meta = with lib; {
+     description = "A Language Server Protocol server for Jsonnet";
+```
+3. Attempt to build the package using `nix build`.
+This command will error but the output will provide expected checksum for the vendored packages.
+```console
+$ cd nix
+$ nix build
+warning: Git tree '/home/jdb/ext/grafana/jsonnet-language-server/jsonnet-language-server' is dirty
+error: hash mismatch in fixed-output derivation '/nix/store/7p66cd269hnsgli1js7hb3lg2498kwfm-jsonnet-language-server-0.6.3-go-modules.drv':
+         specified: sha256-AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA=
+            got:    sha256-mGocX5z3wf9KRhE9leLNCzn8sVdjKJo6FzgP1OwQB3B=
+error: 1 dependencies of derivation '/nix/store/90f116zvv0lx1p15kah9psrb162aarhh-jsonnet-language-server-0.6.3.drv' failed to build
+```
+4. Update the `vendorSha256` with the expected checksum from the previous commands output.
+```patch
+--- a/nix/default.nix
++++ b/nix/default.nix
+@@ -9,7 +9,7 @@ buildGoModule rec {
+     -X main.version=${version}
+   '';
+   src = lib.cleanSource ../.;
+-  vendorSha256 = lib.fakeSha256;
++  vendorSha256 = "sha256-mGocX5z3wf9KRhE9leLNCzn8sVdjKJo6FzgP1OwQB3B=";
+
+   meta = with lib; {
+     description = "A Language Server Protocol server for Jsonnet";
+```
+5. Confirm the build succeeds with `nix build`.
+```console
+$ cd nix
+$ nix build
+$ ./result/bin/jsonnet-language-server --version
+jsonnet-language-server version <major>.<minor>.<patch>
+```
+6. Commit and push the changes.