document converting from git to annex and back

doc/bugs/cannot_commit___34__annex_add__34__ed_modified_file_which_switched_its_largefile_status_to_be_committed_to_git_now/comment_4_8ab0f0852a1dc6c51a502f07faba61eb._comment

@@ -0,0 +1,8 @@
+[[!comment format=mdwn
+ username="joey"
+ subject="""comment 4"""
+ date="2018-08-09T19:04:28Z"
+ content="""
+Documented both conversions in
+<https://git-annex.branchable.com/tips/largefiles>
+"""]]

doc/tips/largefiles.mdwn

@@ -108,3 +108,34 @@ be stored in the annex, you can temporarily override the configuration like
 this:
 
 	git annex add -c annex.largefiles=anything smallfile
+
+## converting git to annexed
+
+When you have a file that is currently stored in git, and you want to
+convert that to be stored in the annex, here's how to accomplish that:
+
+	git rm --cached file
+	git annex add -c annex.largefiles=anything file
+	git commit file
+
+This first removes the file from git's index cache, and then adds it back
+using git-annex. You can modify the file before the `git-annex add` step,
+perhaps replacing it with new larger content that necessitates git-annex.
+
+## converting annexed to git
+
+When you have a file that is currently stored in the annex, and you want to
+convert that to be stored in git, here's how to accomplish that:
+
+	git annex unlock file
+	git add file
+	git commit -n file
+
+You can modify the file after unlocking it and before adding it to
+git. And this is probably a good idea if it was really a big file,
+so that you can replace its content with something smaller.
+
+Notice the `-n` switch when committing the file. This bypasses the
+[[git-annex-precommit]] hook. In this situation, that hook sees an unlocked
+file and wants to add it back to the annex, so you have to bypass the hook
+to get the file committed to git.