.gitattributes: mark '*.md' as Markdown files

Denote Markdown files as such with the 'diff' Git attribute [1]. This
allows various Git commands (diff [2, 3], grep [4, 5], log [6], blame
[7]) to show in which part of the "code" the changes are located in the
hunk header, i.e. the previous commit would be shown as

    diff --git a/docs/remote/troubleshooting.md b/docs/remote/troubleshooting.md
    index bfcf303b7..5ab4f6e60 100644
    --- a/docs/remote/troubleshooting.md
    +++ b/docs/remote/troubleshooting.md
    @@ -516,8 +516,9 @@ ### Cleaning up the VS Code Server on the remote
     If you want to run these steps manually, or if the command isn't working for you, you can run a script like this:

     ```bash
    -kill -9 `ps ax | grep "remoteExtensionHostAgent.js" | grep -v grep | awk '{print $1}'`
    -kill -9 `ps ax | grep "watcherService" | grep -v grep | awk '{print $1}'`
    +# Kill server processes
    +kill -9 `ps aux | \grep vscode-server | \grep USER | \grep -v grep | awk '{print $2}'`
    +# Delete related files and folder
     rm -rf ~/.vscode-server # Or ~/.vscode-server-insiders
     ```

Notice how the hunk header indicates the changes are in the "Cleaning up the VS
Code Server on the remote" section.

The history of that specific entry can now be interrogated using

    git log -L:"Cleaning up":docs/remote/troubleshooting.md

[1] https://git-scm.com/docs/gitattributes#_defining_a_custom_hunk_header
[2] https://git-scm.com/docs/git-diff#_generating_patch_text_with_p
[3] https://git-scm.com/docs/git-diff#Documentation/git-diff.txt--W
[4] https://git-scm.com/docs/git-grep#Documentation/git-grep.txt--p
[5] https://git-scm.com/docs/git-grep#Documentation/git-grep.txt--W
[6] https://git-scm.com/docs/git-log#Documentation/git-log.txt--Lltfuncnamegtltfilegt
[7] https://git-scm.com/docs/git-blame#Documentation/git-blame.txt--Lltfuncnamegt

.gitattributes

@@ -3,3 +3,4 @@
 *.mp4 filter=lfs diff=lfs merge=lfs -text
 *.jpg filter=lfs diff=lfs merge=lfs -text
 *.png filter=lfs diff=lfs merge=lfs -text
+*.md diff=markdown