update docs and include frontend building

relevant: #105

doc/building.md

@@ -5,14 +5,24 @@ Instructions to build the mapserver from source
 
 ## Build dependencies
 
-* go >= 1.11
+* go >= 1.11 (for the binary)
+* rollup >= 1.x (for the embedded js/css assets)
 
 Ubuntu install: https://github.com/golang/go/wiki/Ubuntu
 
 ## Compile
 
+
+Generate the js bundle for the frontend:
+```
+cd static/js
+rollup -c rollup.config.js
+```
+
+Generate the `mapserver` binary:
 ```bash
 # generate the static web files
+# this step embeds the generated js/css/html files for inclusion in the resulting binary
 go generate
 
 # build the binary for the current playtform
@@ -22,3 +32,5 @@ go build
 go test ./...
 
 ```
+
+