Document critical path in perf.md

![trace example](./trace_example.png)

### Critical path

soong_ui logs the wall time of the longest dependency chain compared to the

elapsed wall time in `$OUT_DIR/soong.log`.  For example:

```

critical path took 3m10s

elapsed time 5m16s

perfect parallelism ratio 60%

critical path:

    0:00 build out/target/product/generic_arm64/obj/FAKE/sepolicy_neverallows_intermediates/policy_2.conf

    0:04 build out/target/product/generic_arm64/obj/FAKE/sepolicy_neverallows_intermediates/sepolicy_neverallows

    0:13 build out/target/product/generic_arm64/obj/ETC/plat_sepolicy.cil_intermediates/plat_sepolicy.cil

    0:01 build out/target/product/generic_arm64/obj/ETC/plat_pub_versioned.cil_intermediates/plat_pub_versioned.cil

    0:02 build out/target/product/generic_arm64/obj/ETC/vendor_sepolicy.cil_intermediates/vendor_sepolicy.cil

    0:16 build out/target/product/generic_arm64/obj/ETC/sepolicy_intermediates/sepolicy

    0:00 build out/target/product/generic_arm64/obj/ETC/plat_seapp_contexts_intermediates/plat_seapp_contexts

    0:00 Install: out/target/product/generic_arm64/system/etc/selinux/plat_seapp_contexts

    0:02 build out/target/product/generic_arm64/obj/NOTICE.txt

    0:00 build out/target/product/generic_arm64/obj/NOTICE.xml.gz

    0:00 build out/target/product/generic_arm64/system/etc/NOTICE.xml.gz

    0:01 Installed file list: out/target/product/generic_arm64/installed-files.txt

    1:00 Target system fs image: out/target/product/generic_arm64/obj/PACKAGING/systemimage_intermediates/system.img

    0:01 Install system fs image: out/target/product/generic_arm64/system.img

    0:01 Target vbmeta image: out/target/product/generic_arm64/vbmeta.img

    1:26 Package target files: out/target/product/generic_arm64/obj/PACKAGING/target_files_intermediates/aosp_arm64-target_files-6663974.zip

    0:01 Package: out/target/product/generic_arm64/aosp_arm64-img-6663974.zip

    0:01 Dist: /buildbot/dist_dirs/aosp-master-linux-aosp_arm64-userdebug/6663974/aosp_arm64-img-6663974.zip

```

If the elapsed time is much longer than the critical path then additional

parallelism on the build machine will improve total build times.  If there are

long individual times listed in the critical path then improving build times

for those steps or adjusting dependencies so that those steps can run earlier

in the build graph will improve total build times.

### Soong

Soong can be traced and profiled using the standard Go tools. It understands