Add instructions for adding repos from elsewhere

README.md

@@ -2,3 +2,15 @@
 
 This is a monorepo for all my advent of code solutions;
 each branch is named after its year.
+To add an existing year (20xx):
+
+```sh
+$ git switch main            # root
+$ git branch -c 20xx         # create branch from root named 20xx
+$ git switch 20xx            # switch to 20xx branch
+$ git remote add 20xx <url>  # add remote repo as 20xx
+$ git fetch --all            # 20xx/main points to main branch of 20xx
+$ git rebase 20xx 20xx/main  # rebase remote branch on top of root
+$ git branch 20xx -f         # move branch pointer 20xx to rebased commit
+$ git log                    # take a browse just to check
+```