Change from ffi to ffi-napi since the former seems unsupported

This should also fix installation problem of node-gyp in CI

src/runtime/javascript/README.md

@@ -1,24 +1,32 @@
 # JavaScript (Node.js) bindings to PGF runtime
 
-## Pre-requisites
+## Using in your project
 
+1. You must have installed the PGF C runtime (see `../c/README.md`)
+2. You may need to set the environment variable `LD_LIBRARY_PATH=/usr/local/lib`
+3. Add to your project the usual way with:
+```
+npm install path/to/this/folder
+```
+4. Or install globally with:
+```
+npm install --global .
+```
+
+## Building the package (development)
+
+**Pre-requisites**
 ```
 npm install
 ```
 
-You might need this:
-```
-PYTHON=/usr/bin/python2 npm install -g node-gyp
-```
-
-## Installation
+**Compiling**
 
 ```
 npm run build
 ```
 
-## Running tests
-
+**Running tests**
 ```
 npm run test
 ```