Introduced HACKING.md to document some tools.

HACKING.md

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+
+There are several gradle flags one can use for Konan build.
+
+* **-Pkonanc_flags** passes flags to the compiler used to build stdlib
+
+        ./gradlew -Pkonanc_flags="-disable lower_inline -print_ir" stdlib
+
+* **-Pshims** compiles LLVM interface with tracing "shims". Allowing one 
+    to trace the LLVM calls from the compiler.
+    Make sure to rebuild the project.
+
+        ./gradlew -Pshims=true dist
+
+* **-Pfilter** allows one to choose test files to run.
+
+        ./gradlew -Pfilter=overflowLong.kt run_external
+