update readme with goals and non-goals

README.md

@@ -4,7 +4,22 @@ MadLisp is a [Lisp](https://en.wikipedia.org/wiki/Lisp_%28programming_language%2
 
 ## Goals
 
-The goal of the project was to learn about the internals of programming languages and to build a simple language suitable for scripting and similar use cases.
+* REPL environment where the user can interactively experiment with the language. Suitable for executing pieces of code one by one and examining the internal state of the system.
+* Minimal safeguards or restrictions as to what can be done. Breaking things or using the language in unexpected ways should be part of the fun.
+* Performance does not need to match commercial-grade languages, but needs to be fast enough for real-world programs and uses cases.
+* Suitable to be used as a scripting language in Linux shell scripts and similar environments.
+* Suitable to be used as an embedded scripting language inside another PHP application.
+* Clear and intuitive error messages. This is important for pleasant user experience.
+* Provide a library with commonly used features such as HTTP requests, JSON processing and SQL database support.
+* Provide a clean [interface](src/Lib/ILib.php) for extending the language with your own functions defined in PHP.
+* Provide a safe-mode where access to the file system and other external I/O is restricted.
+* Provide a debug mode which shows what is happening inside the code evaluation.
+* Loosely respect the Lisp legacy with things like naming conventions but do not be constrained by it.
+
+## Non-goals
+
+* Ability to call arbitrary PHP functions directly. The language should have control over which PHP functions can be called and how.
+* Namespaces or similar mechanisms. The global namespace is a feature, not a bug! Use a prefix for your function names if this becomes a problem.
 
 ## Features