improved documentation for installing nodejs

docs/installation.md

@@ -49,7 +49,7 @@ Or use `nohup ./bgpalerter-linux-x64 &` to leave it running after you close the
 
 1. Git clone this repo.
 
-2. Install Node.js (version >= 10.16) and npm ([installing node and npm](https://nodejs.org/en/download/)).
+2. Install Node.js (version >= 10.16) and npm ([installing node and npm](node.md)).
 
 3. Execute `npm install` to install all dependencies.
 

docs/node.md

@@ -0,0 +1,25 @@
+# Installing Node.js
+
+Some fast commands are below. The complete documentation (including other platforms) is [here](https://nodejs.org/en/download/)
+
+## With apt (e.g. debian)
+
+```bash
+curl -sL https://deb.nodesource.com/setup_12.x | sudo bash -
+sudo apt install nodejs
+```
+
+
+## With homebrew (e.g. macos)
+
+```bash
+brew update
+brew install node@12
+```
+
+## With yum (e.g. centos)
+
+```bash
+curl -sL https://deb.nodesource.com/setup_12.x | sudo bash -
+sudo yum install nodejs
+```