Add changelog update info to CONTRIBUTING.md

Author: halfalicious

CONTRIBUTING.md

@@ -3,6 +3,8 @@
 1. All submitted **Pull Requests** are assumed to be ready for review 
    unless they are labeled with `[in progress]` or have "[WIP]" in title.
 
+2. All submitted **Pull Requests** which change Aleth in a way that is visible to users (e.g. new features, bug fixes, significant logging changes) should include an entry in [`CHANGELOG.md`](CHANGELOG.md) in the latest version section. Please categorize your entry appropriately and group it with other entries of the same category. Please see the following for details on categories and how to make a good changelog entry: https://keepachangelog.com/en/1.0.0/
+
 2. **Code formatting** rules are described by the [Clang-Format Style Options] file [.clang-format].
    Please use the [clang-format] (version 5.0 or higher recommended) tool to format your code _changes_ accordingly.
    [git-clang-format] tool is useful to limit reformatting to your changes only:
@@ -16,3 +18,4 @@
 [clang-format]:               https://clang.llvm.org/docs/ClangFormat.html
 [.clang-format]:              .clang-format
 [git-clang-format]:           https://llvm.org/svn/llvm-project/cfe/trunk/tools/clang-format/git-clang-format
+