Add the CHANGELOG.md file

The Changelog.md contains the feature of the first releases. Basically,
it just states out the required configuration to use the color scheme,
and the colors that are used.

CHANGELOG.md

@@ -0,0 +1,39 @@
+# Changelog
+All notable changes to this project will be documented in this file. This
+project adheres to [Semantic Versioning].
+
+## Unreleased
+
+### Added
+
+- Add the majority of the default highlight groups for the syntax and for the
+  editor.
+
+- Add the 12-color palette of the Odyssey color scheme.
+
+  |  Color  |     Main usage    | Hue | Saturation | Value |
+  | ------- | ----------------- | --- | ---------- | ----- |
+  | #0c0f0f |         VertSplit | 177 |     20     |    6  |
+  | #161c1c | Normal background | 177 |     20     |   11  |
+  | #1f2928 |        CursorLine | 177 |     25     |   16  |
+  | #283635 |            Folded | 177 |     25     |   21  |
+  | #808c8c |            LineNr | 177 |      9     |   55  |
+  | #aabfbe | Normal foreground | 177 |     11     |   75  |
+  | #62b3a5 |           Keyword | 170 |     45     |   70  |
+  | #b36262 |          ErrorMsg |   0 |     45     |   70  |
+  | #b3a862 |        WarningMsg |  30 |     45     |   70  |
+  | #62b37d |          Function | 140 |     45     |   70  |
+  | #6298b3 |              Type | 200 |     45     |   70  |
+  | #6270b3 |          Constant | 230 |     45     |   70  |
+
+- Add the prerequisites to use the colorscheme.
+
+  + When Vim is used in a terminal, the 'termguicolors' feature must available
+    and it must be set on. In other words, the color scheme must be used as
+    followed.
+
+  + A ISO-8613-3 compatible terminal is required.
+
+  + Otherwise, a GUI version of Vim must be used.
+
+[Semantic Versioning]: https://semver.org/spec/v2.0.0.html