Update README.md (#413)

Added instructions to clone the most stable release

Author: david-montiel-t

README.md

@@ -48,9 +48,13 @@ Install CMake, p4est, and deal.II (version 9.6 recommended).
 
 Clone the PRISMS-PF GitHub repository and navigate its folder.
 ```bash
-git clone https://github.com/prisms-center/phaseField.git
+git clone -b <tag ID> https://github.com/prisms-center/phaseField.git
 cd phaseField
 ```
+Here <tag ID> corresponds to the stable release version of your choice. For example to clone the latest release (2.4), type
+```bash
+git clone -b v2.4 https://github.com/prisms-center/phaseField.git
+```
 Configure and compile the main library.
 ```bash
 cmake . && make -j <nprocs>