Update Git_commit.md

Git_commit.md

@@ -6,11 +6,47 @@ Initially adapted from a [blog post](https://chris.beams.io/posts/git-commit/).
 
 We want commit messages to be concise and consistent, to communicate the context of a change. 
 
-  * The `diff` contains *what* changes were made, the `commit` explains *why* they were made.
+  * The `diff` contains *what* and *how* changes were made, the `commit` primarily explains *why* they were made.
   * Consistent commit message style 
       * simplifies both reading and writing commit messages, and
       * supports communication and collaboration.
 
+## Context
+
+There are different ways and contexts whereby a commit message is added, but for simplicity we assume you made some changes, and did the following commands in the terminal:
+```
+git add .
+git commit
+```
+
+You will then have vi open and provide the following starting point:
+```
+
+# Please enter the commit message for your changes. Lines starting
+# with '#' will be ignored, and an empty message aborts the commit.
+#
+# On branch master
+# Your branch is up to date with 'origin/master'.
+#
+# Changes to be committed:
+#       new file:   foo
+#
+```
+
+And then your job is to write a good commit message! Document why you are making the commit and the associated changes.
+
+## Rules
+
+You should at least write a single line if your commit only needs a brief explanation. We'll call this the subject, whether or not you also write a longer text, called the body, below the subject. 
+
+### Subject
+
+The subject should be
+  * captialized, e.g. *Combine the Christmas lighbulbs*,
+  * maximum 50 characters,
+  * written with no terminating period, and
+  * the subject should be written in the *imperative mood*.
+
 ## Writing style conventions
 
   * Write complete and grammatical sentences in English, with regular punctuation, e.g, finishing with a period.