Convert ChangeLog.d/README to markdown

Signed-off-by: Gilles Peskine <Gilles.Peskine@arm.com>
diff --git a/ChangeLog.d/00README.md b/ChangeLog.d/00README.md
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index 0000000..774460d
--- /dev/null
+++ b/ChangeLog.d/00README.md
@@ -0,0 +1,20 @@
+This directory contains changelog entries that have not yet been merged
+to the changelog file ([`../ChangeLog`](../ChangeLog)).
+
+A changelog entry file must have the extension `*.txt` and must have the
+following format:
+
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+Security
+   * Change description.
+   * Another change description.
+
+Features
+   * Yet another change description.
+   * Yet again another change description.
+
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+See `STANDARD_CATEGORIES` in
+[`../scripts/assemble_changelog.py`](../scripts/assemble_changelog.py)
+for recognized section titles.
diff --git a/ChangeLog.d/README b/ChangeLog.d/README
deleted file mode 100644
index 6e8e309..0000000
--- a/ChangeLog.d/README
+++ /dev/null
@@ -1,19 +0,0 @@
-This directory contains changelog entries that have not yet been merged
-to the changelog file (../ChangeLog).
-
-A changelog entry file must have the extension *.txt and must have the
-following format:
-
-~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
-Security
-   * Change description.
-   * Another change description.
-
-Features
-   * Yet another change description.
-   * Yet again another change description.
-
-~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
-
-See STANDARD_SECTIONS in ../scripts/assemble_changelog.py for
-recognized section titles.