Update README.md

Closes #21

README.md

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-# congenial-elucidator
+# congenial-elucidator
+congenial-elucidator is a compiler for the xpln programming language, targeting MIPS architecture.
+Actually, for a slightly modified version of xpln, targeting SPIM simulator.
+
+## yalalr
+congenial-elucidator is written using (yalalr)[https://github.com/bozsahin/yalalr].
+See the instructions there for usage.
+
+## Example programs
+Under the `xpln/` directory, there are sample xpln programs.
+Some of them are taken from (Berk Özbalcı's xpln stress test)[https://gist.github.com/bozbalci/13bae202aab2fbe50be2bc474fc82292].
+Under `mips/`, there are target codes for each of the example programs, as compiled by congenial-elucidator.
+
+## What's missing
+* Sadly, no floating point support. Integers only.
+
+## What's different from original specs
+* Function definitions at the top: Grammar was slightly modified to simplify code generation,
+in the new grammar function definitions come before the main body and there can be no
+function definitions between statements.
+
+## What can be improved
+* Determine stack frame size dynamically. Currently, all functions have a fixed
+stack frame size, which is 512 bytes. Ideally, we should determine at compile time
+how much stack space to allocate for each function.
+* Use unsigned arithmetic for address computations.
+
+## Known problems
+* Grammar is not LALR(1). It haven't caused any harm (yet) though; I think
+LALR parser generator's default decisions save me there.
+* Return value of main is ignored. I couldn't find out how to return a value from
+a program in SPIM. I don't know if it's possible.
+
+## Acknowledgements
+I would like to thank Prof. Dr. Cem Bozşahin (@bozsahin) for the awesome course!
+Actually, a "course" would be an understatement for this; it was more like
+a journey from heaven to hell and back to heaven again, all in one semester.
+Also see (bozsahin/ceng444)[https://github.com/bozsahin/ceng444].
+
+I would also like to thank Berk Özbalcı (@bozbalci) for his friendship and support,
+and also his (xpln stress test)[https://gist.github.com/bozbalci/13bae202aab2fbe50be2bc474fc82292].