README.md: wrap lines to under 100 characters

Author: maxim-belkin

README.md

@@ -1,5 +1,5 @@
-[![Create a Slack Account with us](https://img.shields.io/badge/Create_Slack_Account-The_Carpentries-071159.svg)](https://swc-slack-invite.herokuapp.com/)
-[![Slack Status](https://img.shields.io/badge/Slack_Channel-dc--ecology--py-E01563.svg)](https://swcarpentry.slack.com/messages/C9X44HCDS)
+[![Create a Slack Account with us][slack-account-icon]][slack-heroku]
+[![Slack Status][slack-status-icon]][slack-status]
 
 ## Data Carpentry Python Lessons with Ecological Data
 
@@ -9,16 +9,16 @@ on how to contribute updates, bug fixes, or other corrections.
 
 ## Contributing
 
-We welcome all contributions to improve the lesson! Maintainers will do their best to help you if you have any
-questions, concerns, or experience any difficulties along the way.
+We welcome all contributions to improve the lesson! Maintainers will do their best to help you
+if you have any questions, concerns, or experience any difficulties along the way.
 
-We'd like to ask you to familiarize yourself with our [Contribution Guide](CONTRIBUTING.md) and have a look at
-the [more detailed guidelines][lesson-example] on proper formatting, ways to render the lesson locally, and even
-how to write new episodes.
+We'd like to ask you to familiarize yourself with our [Contribution Guide](CONTRIBUTING.md)
+and have a look at the [more detailed guidelines][lesson-example] on proper formatting,
+ways to render the lesson locally, and even how to write new episodes.
 
 Please see the current list of [issues][issues] for ideas for contributing to this
 repository. For making your contribution, we use the GitHub flow, which is
-nicely explained in the chapter [Contributing to a Project](http://git-scm.com/book/en/v2/GitHub-Contributing-to-a-Project) in Pro Git
+nicely explained in the chapter [Contributing to a Project][contributing-to-a-project] in Pro Git
 by Scott Chacon.
 Look for the tag [![Image replicating 'Good first issue' label][gfi-label]][gfi].
 This indicates that the maintainers will welcome a pull request fixing this issue.
@@ -38,8 +38,13 @@ A list of contributors to the lesson can be found in [AUTHORS](AUTHORS)
 
 To cite this lesson, please consult with [CITATION](CITATION)
 
-[lesson-example]: https://carpentries.github.io/lesson-example
-[maxim-belkin]: https://github.com/maxim-belkin
-[issues]: https://github.com/datacarpentry/python-ecology-lesson/issues
+[contributing-to-a-project]: http://git-scm.com/book/en/v2/GitHub-Contributing-to-a-Project
 [gfi-label]: https://img.shields.io/badge/-good%20first%20issue-gold.svg
 [gfi]: https://github.com/datacarpentry/python-ecology-lesson/labels/good%20first%20issue
+[issues]: https://github.com/datacarpentry/python-ecology-lesson/issues
+[lesson-example]: https://carpentries.github.io/lesson-example
+[maxim-belkin]: https://github.com/maxim-belkin
+[slack-account-icon]: https://img.shields.io/badge/Create_Slack_Account-The_Carpentries-071159.svg
+[slack-heroku]: https://swc-slack-invite.herokuapp.com/
+[slack-status-icon]: https://img.shields.io/badge/Slack_Channel-dc--ecology--py-E01563.svg
+[slack-status]: https://swcarpentry.slack.com/messages/C9X44HCDS