Add Zsh and Bash syntax highlighting to README.md

Verified exhaustiveness thanks to:

```bash
grep -r '```' --include=*.md
```

README.md

@@ -48,15 +48,15 @@ $ pip install mysqlclient
 
 Install MySQL and mysqlclient:
 
-```
+```zsh
 # Assume you are activating Python 3 venv
 $ brew install mysql pkg-config
 $ pip install mysqlclient
 ```
 
 If you don't want to install MySQL server, you can use mysql-client instead:
 
-```
+```zsh
 # Assume you are activating Python 3 venv
 $ brew install mysql-client pkg-config
 $ export PKG_CONFIG_PATH="/opt/homebrew/opt/mysql-client/lib/pkgconfig"
@@ -88,7 +88,7 @@ compiler/linker flags.
 You can use `MYSQLCLIENT_CFLAGS` and `MYSQLCLIENT_LDFLAGS` environment
 variables to customize compiler/linker options.
 
-```
+```bash
 $ export MYSQLCLIENT_CFLAGS=`pkg-config mysqlclient --cflags`
 $ export MYSQLCLIENT_LDFLAGS=`pkg-config mysqlclient --libs`
 $ pip install mysqlclient