Add doc for thinlto corpus extraction (#111)

Author: Northbadge

docs/thinlto.md

@@ -0,0 +1,42 @@
+# Extracting Corpora for ThinLTO-enabled Projects
+
+These steps use inlining for size as an example.
+
+## Generate the corpus
+
+Pass `-thinlto-emit-index-files -save-temps=import` to lld, this will output `.3.import.bc`
+and `.thinlto.bc` files in your source directory.   
+
+## Consolidate the corpus
+
+Run `tools/extract_ir.py` with `--thinlto_build=local` and set `--obj_base_dir`, `--output_dir`
+accordingly.
+
+## Modify corpus_description.json
+
+Modification of `corpus_description.json` which is written to `output_dir` is necessary. An error will
+be thrown if this is not done. \
+The `global_command_override` field in the json needs to be filled with the options to run
+**clang** with on each module. These options should be inferred from the lld command that generated the
+corpus. Most importantly, it should include "-c", some "-O" flag, relevant mllvm flags, and target/arch
+flags. \
+\
+Here's an example based on Chrome:
+```json
+{
+  "global_command_override": [
+    "-fPIC",
+    "-mllvm",
+    "-instcombine-lower-dbg-declare=0",
+    "-mllvm",
+    "-import-instr-limit=5",
+    "-march=armv7-a",
+    "--target=arm-linux-androideabi23",
+    "-no-canonical-prefixes",
+    "-O2",
+    "-nostdlib++",
+    "--sysroot=/path/to/linux-x86_64/sysroot",
+    "-c"
+  ]
+}
+```