[BOLT][docs] Document commands to identify BOLT-ed binaries (#171458)

These have been useful to me in the past and I don't see any docs
referencing the `bolt_info` note either.

There is a test checking that it is emitted, so I assume it's something
we are ok with people looking for.

Author: DavidSpickett

bolt/README.md

@@ -202,6 +202,26 @@ $ merge-fdata *.fdata > combined.fdata
 Use `combined.fdata` for **Step 3** above to generate a universally optimized
 binary.
 
+## Identifying a Binary Modified By BOLT
+
+A binary that has been modified by BOLT will include a `bolt_info` note and may
+have extra sections with `bolt` in their name.
+
+You can use `readelf` to find these:
+```
+$ readelf -S <your-binary> | grep bolt
+  [11] .bolt.org.eh_frame PROGBITS <...>
+<...>
+  [39] .note.bolt_info   NOTE <...>
+```
+The note can be displayed with:
+```
+$ readelf -p .note.bolt_info <your-binary>
+String dump of section '.note.bolt_info':
+  <...>
+  [    10]  BOLT revision: <...>
+```
+
 ## License
 
 BOLT is licensed under the [Apache License v2.0 with LLVM Exceptions](./LICENSE.TXT).