Add section on commit messages to coding style

Also enables the table of contents for easier navigation. Adds an
explicit header ID to be able to reference it later. Both changes
could cause trouble in earlier versions of Doxygen. Tested with
version 1.8.20.

Signed-off-by: Daniel Sieger <[email protected]>

docs/codingstyle.md

@@ -1,5 +1,7 @@
 # Coding Style {#codingstyle}
 
+[TOC]
+
 This section describes some of the coding standards that are being used in the
 pmp-library source tree. Although these standards should never be regarded as
 strict requirements they are still important to establish a minimum level of
@@ -289,6 +291,38 @@ m(3, 0) = 0.0;  m(3, 1) = 0.0;  m(3, 2) = 0.0;  m(3, 3) = 1.0;
 // clang-format on
 ```
 
+## Commit Messages {#commit-messages}
+
+Please write meaningful commit messages. See [this post](https://chris.beams.io/posts/git-commit/) for the why and how.
+
+The basic rules are:
+
+1. Separate subject from body with a blank line
+2. Limit the subject line to 50 characters
+3. Capitalize the subject line
+4. Do not end the subject line with a period
+5. Use the imperative mood in the subject line
+6. Wrap the body at 72 characters
+7. Use the body to explain what and why vs. how
+
+This is what an example might look like:
+
+```text
+Summarize changes in around 50 characters or less
+
+More detailed explanatory text, if necessary. Wrap it to about 72
+characters or so. Explain the problem this commit is solving.
+Focus on why you are making this change as opposed to how. Are there
+any side effects or counter-intuitive consequences?
+
+Reference issues at the bottom, sign off on last line:
+
+Resolves: #123
+See also: #456, #789
+
+Sign-off-by: Jane Miller <[email protected]>
+```
+
 ## See Also
 
 The above coding standard is necessarily incomplete. Therefore, we list some