Brush up the go code

README.md

@@ -2,19 +2,46 @@
 
 Just generates some magic PDF invoice covers using straight up dark magic.
 
-## Just be lazy
+## Building it
 
-Go see it in action and download it from [https://s3.blacknova.io/covergen/index.html](https://s3.blacknova.io/covergen/index.html)
+```shell
+# Builds the CLI
+make build
 
-## Just build it
-
-```
-$ go build -o covergen ./cmd/covergen
+# Builds the WASM blob
+make wasm
 ```
 
-## It has wasm!
+## Custom font notes
 
-```
-$ make wasm
+The PDF uses the font `Mark`, which isn't a commonly available font.
+For this reason they have been vendored into the repository in `pkg/covergen/fonts` together with
+the `cp1252` character map (used in most PDFs because UTF-8 hasn't been invented yet in the PDF world).
+For usage in `fpdf` they need to be encoded, which is done throught he `makefont` tool provided by `fpdf`, see
+[https://github.com/go-pdf/fpdf#nonstandard-fonts](https://github.com/go-pdf/fpdf#nonstandard-fonts).
+The section linked will also provide instructions on how to use it, but for this specific repo, the call to regenerate
+or add more fonts is:
+```shell
+makefont --embed --enc=pkg/covergen/fonts/cp1252.map --dst=pkg/covergen/font_embed pkg/covergen/fonts/*.ttf
 ```
 
+To get the `makefont` binary, clone the `go-fpdf/fpdf` repo and run `go build` in the `makefont` directory.
+
+## A note on WASM
+
+An interesting architectural note about how WASM binaries are run in browsers is that you should not see it
+as a shell command 'exec'-ing out into a new binary, but rather as two processes living side by side.
+The calling a method exposed by WASM behaves more like doing IPC between the two processes.
+You can sort of see this happening in `cmd/wasm/main.go` where the `main` function is actually used.
+It registers the two exposed commands to the `global` scope (in browsers: `window`) before just deadlocking
+by waiting on an anonymous channel.
+
+The most important note you should take from that is that the WASM binary **can crash**.
+If it crashes, it doesn't just 'boot itself' again. 
+And the JS -> WASM boundary is *very*, ***very*** unsafe. 
+Like 'do the smallest thing wrong and the program crashes' level of unsafe.
+No, type safety won't help you, you're literally playing with the equivalent of void pointers.
+Even panic recovery (present in the two exposed methods) doesn't protect against all of it.
+
+I think where I'm going is just a strong warning to be *very* careful at that boundary.
+Don't trust any `js.Value`. Parse it thoroughly and quickly into golang-specified types.