Describe githubGenerateSnapshot in readme

Author: adpi2

README.md

@@ -107,6 +107,32 @@ steps:
 
 ## Troubleshooting
 
+### How to generate a snapshot locally?
+
+For troubleshooting, it can be convenient to generate a snapshot locally.
+
+To do so you need to install the `sbt-dependency-submission` plugin in your sbt project.
+
+```scala
+// In project/plugins.sbt
+addSbtPlugin("ch.epfl.scala" % "sbt-github-dependency-submission" % "3.0.0")
+```
+
+After reloading your build, you can run:
+```
+sbt:example> githubGenerateSnapshot
+...
+[info] Dependency snapshot written to /tmp/dependency-snapshot-3080240838874963577.json
+```
+
+Or if you want to exclude some modules or configs:
+
+```
+sbt:example> githubGenerateSnapshot {"ignoredModules":["server_2.13"], "ignoredConfigs":["test"]}
+...
+[info] Dependency snapshot written to /tmp/dependency-snapshot-14803616116503623758.json
+```
+
 ### Unexpected Status: 404
 
 This error happens when the `Dependency Graph` feature is disabled.

sbt-plugin/src/main/scala/ch/epfl/scala/SubmitDependencyGraph.scala

@@ -22,7 +22,7 @@ import sjsonnew.support.scalajson.unsafe.{Parser => JsonParser, _}
 
 object SubmitDependencyGraph {
   val Generate = "githubGenerateSnapshot"
-  private val GenerateUsage = s"""$Generate {"projects":[], "scalaVersions":[]}"""
+  private val GenerateUsage = s"""$Generate {"ignoredModules":[], "ignoredConfig":[]}"""
   private val GenerateDetail = "Generate the dependency graph of a set of projects and scala versions"
 
   private val GenerateInternal = s"${Generate}Internal"
@@ -31,8 +31,6 @@ object SubmitDependencyGraph {
   val Submit = "githubSubmitSnapshot"
   private val SubmitDetail = "Submit the dependency graph to Github Dependency API."
 
-  def usage(command: String): String = s"""$command {"projects":[], "scalaVersions":[]}"""
-
   val commands: Seq[Command] = Seq(
     Command(Generate, (GenerateUsage, GenerateDetail), GenerateDetail)(inputParser)(generate),
     Command.command(GenerateInternal, InternalOnly, InternalOnly)(generateInternal),
@@ -43,8 +41,10 @@ object SubmitDependencyGraph {
 
   private def inputParser(state: State): Parser[DependencySnapshotInput] =
     Parsers.any.*.map { raw =>
-      JsonParser
-        .parseFromString(raw.mkString)
+      val rawString = raw.mkString
+      if (rawString.isEmpty) DependencySnapshotInput(None, Vector.empty, Vector.empty)
+      else JsonParser
+        .parseFromString(rawString)
         .flatMap(Converter.fromJson[DependencySnapshotInput])
         .get
     }.failOnException