desktop-pwas: add note to launcher README

Adds a note about component builds to the chrome_pwa_launcher.exe
README.

PWA file handling does not currently work with a component-build
launcher, because it depends on DLLs in the out directory. This makes
that limitation more clear.

Bug: 1123018
Change-Id: I9c5de4fcdc58f0cae1244b96bd152d9cdf12d300
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2382052Reviewed-by: David Bienvenu <davidbienvenu@chromium.org>
Commit-Queue: Jesse McKenna <jessemckenna@google.com>
Cr-Commit-Position: refs/heads/master@{#802715}

chrome/browser/web_applications/chrome_pwa_launcher/README.md

@@ -30,3 +30,7 @@ chrome_pwa_launcher.exe assumes that it is run from a web-app data directory
 (User Data\<profile>\Web Applications\<app ID>), as it uses the "Last Browser"
 file in its great-grandparent User Data directory to find the chrome.exe to
 launch.
+
+NOTE: PWA launchers must be compiled in a non-component build to support file
+handling. Component-build EXEs will hit a missing-DLL error if run from the
+web-app directory.