Add usage of git subtree to readme

Author: antoyo

Readme.md

@@ -193,7 +193,7 @@ Using git-subtree with `rustc` requires a patched git to make it work.
 The PR that is needed is [here](https://github.com/gitgitgadget/git/pull/493).
 Use the following instructions to install it:
 
-```
+```bash
 git clone git@github.com:tqc/git.git
 cd git
 git checkout tqc/subtree
@@ -204,6 +204,12 @@ make
 cp git-subtree ~/bin
 ```
 
+Then, do a sync with this command:
+
+```bash
+PATH="$HOME/bin:$PATH" ~/bin/git-subtree push -P compiler/rustc_codegen_gcc/ ../rustc_codegen_gcc/ sync_branch_name
+```
+
 ### How to use [mem-trace](https://github.com/antoyo/mem-trace)
 
 `rustc` needs to be built without `jemalloc` so that `mem-trace` can overload `malloc` since `jemalloc` is linked statically, so a `LD_PRELOAD`-ed library won't a chance to intercept the calls to `malloc`.