Document mechanism to locally run and test

Author: Oscar Scholten

.gitignore

@@ -1,2 +1,5 @@
 .idea/
-*.iml
+*.iml
+node_modules/
+package-lock.json
+_book/

README-local.txt

@@ -0,0 +1,20 @@
+To locally compile and view the reference guide, run these commands:
+
+$ npm install gitbook-cli
+
+(npm will complain there is no package.json file, that can be ignored)
+
+$ ./node_modules/gitbook-cli/bin/gitbook.js install
+
+This installs the necessary plugins and the latest gitbook version
+
+$ ./node_modules/gitbook-cli/bin/gitbook.js build
+
+This builds the book into the _book folder
+
+Alternatively, you can use:
+
+$ ./node_modules/gitbook-cli/bin/gitbook.js serve
+
+This starts a webserver at port 4000; it also monitors your .md files for
+changes, rebuilds the book and refreshes your browser tab.