📚 Add file with commit message guidelines

Author: ronisbr

COMMITS.md

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+# Commits
+
+This file describe rules that must be applied to commit code to
+ReferenceFrameRotations.jl.
+
+Notice that this document is **under construction**.
+
+## Best practices
+
+The following list of best practices should be considered when creating commits
+for this project:
+
+1. The commits should be incremental. Try to avoid very big changes within just
+   one commit.
+2. The overall state of the software must be as functional as possible between
+   two commits. Try to avoid commits that break the entire software if another
+   commit is not applied.
+
+## Commit message
+
+The commit message must have the following structure:
+
+```
+<EMOJI> Short summary of changes
+                         -- BLANK LINE -- 
+More detailed explanatory text, if necessary.  Wrap it to 67
+characters.  In some contexts, the first line is treated as the
+subject of an email and the rest of the text as the body.  The
+blank line separating the summary from the body is critical (unless
+you omit the body entirely); tools like rebase can get confused if
+you run the two together.
+
+Further paragraphs come after blank lines.
+
+  - Bullet points are okay, too.
+
+  - Typically a hyphen or asterisk is used for the bullet, preceded
+    by a single space, with blank lines in between, but conventions
+    vary here.
+```
+
+**Source**: Adapted from http://git-scm.com/book/ch5-2.html
+
+Notice that:
+
+1. The short summary **must** be less than **50 characters**. The emojis, if
+   present, count as 1 character each.
+2. **Don't** end the summary with a period.
+3. If the summary indicates an action, then if **must** be written in imperative
+   form. Hence, write "Fix ...", "Change ...", and "Add ...", instead of "Fixed
+   ...", "Changed ...", or "Added ...".
+3. It is preferred that only one emoji is used per commit.
+4. All the phrases in the explanatory text **must** be punctuated.
+5. If there is an explanatory text, a blank line **must** exist between the
+   short summary of changes and the explanatory text.
+6. The is no limit for the detailed explanatory text, but it must be wrap to 72
+   characters.
+
+## Commit emojis
+
+Emojis can be used to improve the understanding about the commit. It can be used
+at the beginning of the commit message to indicate what kind of modification the
+commit does. The following table describe some situations:
+
+| Commit Type            | Emoji                                                          |
+|------------------------|----------------------------------------------------------------|
+| Initial Commit         | :tada: `:tada:`                                                |
+| Version Tag            | :bookmark: `:bookmark:`                                        |
+| New Feature            | :sparkles: `:sparkles:`                                        |
+| Bugfix                 | :bug: `:bug:`                                                  |
+| Refactoring            | :package: `:package:`                                          |
+| Documentation          | :books: `:books:`                                              |
+| Internationalization   | :globe_with_meridians: `:globe_with_meridians:`                |
+| Performance            | :racehorse: `:racehorse:`                                      |
+| Cosmetic               | :lipstick: `:lipstick:`                                        |
+| General improvements   | :wrench: `:wrench:`                                            |
+| Tests                  | :rotating_light: `:rotating_light:`                            |
+| Deprecation            | :poop: `:poop:`                                                |
+| Work In Progress (WIP) | :construction: `:construction:`                                |
+| Warning                | :warning: `:warning:`                                          |
+| Other                  | [See here](https://www.webpagefx.com/tools/emoji-cheat-sheet/) |
+
+**Note**: If the commit contains an important warning, such as a breaking change, then the emoji :warning: `:warning:` must be added to the commit title. In this case, two emojis can be used.