Update README.md

Author: paf31

README.md

@@ -2,6 +2,8 @@
 
 Very basic web service which wraps the PureScript compiler.
 
+[Client code](https://github.com/purescript/trypurescript/tree/gh-pages)
+
 ## API
 
 ### Compile PureScript code
@@ -13,3 +15,25 @@ Very basic web service which wraps the PureScript compiler.
 - Status code: 200 (success)
 
 The response does not use error codes, to make it easier to use the API from another domain using CORS.
+
+The output code will contain references to preloaded modules using `require` calls. To run these files in the browser, it is necessary to either use a `require` shim (such as require1k), or replace these calls and deploy a bundle of precompiled modules (the Try PureScript client uses the second approach).
+
+## Configuration
+
+The application takes three arguments on the command line:
+
+- externs path
+- externs config file
+- port number
+
+### Externs Path
+
+This directory should contain externs files for all modules which should be precompiled on startup. The name of each file in the directory should be `Module.Name.json`.
+
+### Externs Config File
+
+An example file is provided in `conf/core.conf`. A config file consists of a list of module names, one per line, topologically sorted by module dependencies and including all trasitive dependencies. The easist way to create this file is to copy the output of `psc` after a fresh compile.
+
+### Example
+
+    dist/build/trypurescript/trypurescript externs/ conf/core.conf 8081