Add .gitattributes file for project (#974)

IMHO it's useful to see Markdown files as one of the types of files
listed on the GitHub front page in the file statistics section,
alongside other files. It gives a sense for the amount of documentation
present in a project.

On the other hand, some configuration files are not useful to count,
such as .md files in a `.gemini/` subdirectory and configuration files
for tools like git.

Author: mhucka

.gitattributes

@@ -0,0 +1,22 @@
+# Copyright 2025 Google LLC
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     https://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+# Include Markdown files in GitHub's file statistics for this repo.
+*.md linguist-detectable
+
+# Exclude config files from GitHub's file statistics. GitHub's Linguist already
+# recognizes most config files as data; this adds some it doesn't.
+.gemini/**             linguist-documentation
+.git-blame-ignore-revs linguist-documentation
+.markdownlintrc        linguist-documentation