update readme

README.md

@@ -1,3 +1,7 @@
+# Website
+
+https://delbetu.github.io/prawn_box/
+
 # Getting started
 
 To run the application locally, you should:
@@ -7,3 +11,35 @@ To run the application locally, you should:
 * `bundle exec rake build_site` to build the site inti `./dist`
 * `ruby -run -e httpd './dist' -p 8000` serve the site from dist.
 * Open a browser at `localhost:8000`.
+
+
+# Contributing
+
+The system is composed of:
+
+app.wasm    ==> ruby interpreter + gems installed into a virtual file system and compiled to wasm
+Gemfile     ==> Hold dependencies (prawn, prawn-table) to be included in app.wasm
+Rakefile    ==> app.wasm task holds the building process for app.wasm
+index.html  ==> UI (creates a webassembly module for app.wasm using ruby-wasm-wasi npm package from ruby.wasm)
+
+## How pdf generation works
+The main view needs to eval ruby code using the prawn library.
+
+```ruby
+require "rubygems"
+$:.unshift("/lib") # virtual file system has this folder with prawn and prawn-table
+require "prawn"
+require "prawn/table"
+
+pdf = Prawn::Document.new
+pdf.instance_eval do
+  text "Hello World!"
+end
+pdf.render
+```
+
+For that, it reads the html input (user ruby code) and calls RubyVm.eval(...).
+The RubyVm actually is a prepared environment containing.
+The interpreter `ruby.wasm` and other gems (`prawn` and `prawn-table`).
+This ruby.wasm executes the ruby code and returns a binary string (the pdf)
+index.html receives that string and puts it that into the `<embed>` tag.