Ignore .pyc files in third_party/[jinja2|markupsafe]

Also update the README.chromium to list the files added in these
directories.

Fixed: dawn:2301
Change-Id: Ia70528d890fb479dc17620cb06fc771116074c76
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5172588
Reviewed-by: Yuki Shiino <yukishiino@chromium.org>
Auto-Submit: Corentin Wallez <cwallez@chromium.org>
Commit-Queue: Yuki Shiino <yukishiino@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1244448}
NOKEYCHECK=True
GitOrigin-RevId: da8822f42d506ad3a1968af90a931191195389af
diff --git a/.gitignore b/.gitignore
new file mode 100644
index 0000000..5727c5e
--- /dev/null
+++ b/.gitignore
@@ -0,0 +1 @@
+/**/*.pyc
diff --git a/README.chromium b/README.chromium
index 1517f33..6242112 100644
--- a/README.chromium
+++ b/README.chromium
@@ -23,4 +23,9 @@
 also posted on website, SHA-512 computed locally); script checks hash then
 unpacks archive and installs desired files.
 Retrieve or update by executing markupsafe/get_markupsafe.sh from third_party.
-We also add markupsafe.gni to list sources.
\ No newline at end of file
+Additional chromium-specific files are:
+* README.chromium (this file)
+* OWNERS
+* DIR_METADATA
+* markupsafe.gni
+* .gitignore