README.md: on install, explicitly instruct to clone the "master" branch

The install instructions relied on the default branch being "master" on Github.
This has no longer been the case for some time now.

Let's be explicit, and directly advise new users to put themselves on the stable
branch ("master").

README.md

@@ -15,7 +15,7 @@ Etherpad is a really-real time collaborative editor scalable to thousands of sim
 ```
 curl -sL https://deb.nodesource.com/setup_9.x | sudo -E bash -
 sudo apt-get install -y nodejs
-git clone https://github.com/ether/etherpad-lite.git && cd etherpad-lite && bin/run.sh
+git clone --branch master https://github.com/ether/etherpad-lite.git && cd etherpad-lite && bin/run.sh
 ```
 
 ## GNU/Linux and other UNIX-like systems
@@ -23,7 +23,7 @@ You'll need git and [node.js](https://nodejs.org) installed (minimum required No
 
 **As any user (we recommend creating a separate user called etherpad):**
 
-1. Move to a folder where you want to install Etherpad. Clone the git repository: `git clone git://github.com/ether/etherpad-lite.git`
+1. Move to a folder where you want to install Etherpad. Clone the git repository: `git clone --branch master git://github.com/ether/etherpad-lite.git`
 2. Change into the new directory containing the cloned source code: `cd etherpad-lite`
 3. run `bin/run.sh` and open <http://127.0.0.1:9001> in your browser.
 
@@ -46,7 +46,7 @@ You'll need [node.js](https://nodejs.org) and (optionally, though recommended) g
 
 1. Grab the source, either
   - download <https://github.com/ether/etherpad-lite/zipball/master>
-  - or `git clone https://github.com/ether/etherpad-lite.git` (for this you need git, obviously)
+  - or `git clone --branch master https://github.com/ether/etherpad-lite.git` (for this you need git, obviously)
 2. start `bin\installOnWindows.bat`
 
 Now, run `start.bat` and open <http://localhost:9001> in your browser.