add migration instructions

MigrateFromFileTree.md

@@ -0,0 +1,44 @@
+Migrate from FileTree to Tonel
+===
+
+I prepared this small script to drive your migration:
+
+```Smalltalk
+locationDir := 'path/to/your/repo' asFileReference.
+subDir := 'your-source-dir-or-empty'.
+sourceDir := locationDir.
+subDir 
+	ifNotNil: [
+		subDirWithDelim := subDir, '/'. 
+		sourceDir := sourceDir / subDir ]
+	ifNil: [
+		subDirWithDelim := '' ].
+
+repo := IceRepositoryCreator new 
+	location: locationDir;
+	subdirectory: subDir;
+	createRepository.
+
+"branch if you want to perform the migration on separated place (you 
+can later do a PR)"
+repo createBranch: 'migrate-sources-to-tonel'.
+
+commit := repo branch lastCommit.
+repo savedPackages do: [ :each | | filetreePackage |
+	TonelWriter fileOut: (commit versionFor: each) on: sourceDir.
+	filetreePackage := IceLibgitFiletreeWriter directoryNameFor: each.
+	(sourceDir / filetreePackage) ensureDeleteAll.
+	 repo addFilesToIndex: { 
+		subDirWithDelim, (IceLibgitTonelWriter directoryNameFor: each).
+		subDirWithDelim, (IceLibgitFiletreeWriter directoryNameFor: each). } ].
+
+repo addProperties: (IceRepositoryProperties fromDictionary: { #format -> #tonel } asDictionary).
+
+repo 
+	commitIndexWithMessage: 'sources migrated' 
+	andParents: { commit }.
+	
+repo push.
+```
+
+*Please note that is not possible to migrate history from FileTree to Tonel. This is because FileTree is file-per-method and Tonel is File-Per-Class, so no conversion can be made (at least that I can think of).*