[lit] Add instructions to run lit's test suite

- Patch by Brian Gesiak.
 
 - https://reviews.llvm.org/D24968

llvm-svn: 282525

llvm/utils/lit/README.txt

@@ -6,3 +6,19 @@ lit is a portable tool for executing LLVM and Clang style test suites,
 summarizing their results, and providing indication of failures. lit is designed
 to be a lightweight testing tool with as simple a user interface as possible.
 
+=====================
+ Contributing to lit
+=====================
+
+Please read the TODO file in this directory for ideas on what to work on.
+Before submitting patches, run the test suite to ensure nothing has regressed:
+
+    # From within your LLVM source directory.
+    utils/lit/lit.py \
+        --path /path/to/your/llvm/build/bin \
+        utils/lit/tests
+
+Note that lit's tests depend on 'not' and 'FileCheck', LLVM utilities.
+You will need to have built LLVM tools in order to run lit's test suite
+successfully.
+