Makefile: better "github-ssh" target

Makefile

@@ -149,10 +149,8 @@ mrproper: clean
 
 # for when you want to use SSH keys with GitHub
 github-ssh:
-	sed -i 's#https://github.com/#git@github.com:#' .git/config
-	git config --file .gitmodules --get-regexp url | while read LINE; do \
-		git config `echo $${LINE} | sed 's#https://github.com/#git@github.com:#'` \
-		;done
+	git config url."git@github.com:".insteadOf "https://github.com/"
+	git submodule foreach --recursive 'git config url."git@github.com:".insteadOf "https://github.com/"'
 
 #- re-builds the Nim compiler (not usually needed, because `make update` does it when necessary)
 #- allows parallel building with the '+' prefix

README.md

@@ -108,6 +108,12 @@ make V=0 # quiet
 make V=2 test # more verbose than usual
 ```
 
+- if you want to use SSH keys with GitHub:
+
+```bash
+make github-ssh
+```
+
 #### Git submodule workflow
 
 Working on a dependency: