Create CONTRIBUTING.md

CONTRIBUTING.md

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+# Contributing to GrimoireLab Toolkit
+
+There are some general guidelines and information related to how we 
+contribute to GrimoireLab. You can know more about them from here, 
+[CONTRIBUTING.md](https://github.com/chaoss/grimoirelab/blob/master/CONTRIBUTING.md).
+
+Toolkit is one of the components in the 
+[GrimoireLab toolset](https://github.com/chaoss/grimoirelab#grimoirelab-components) 
+which stores the common functions used across GrimoireLab projects.
+
+## Releases
+
+GrimoireLab will deliver frequent coordinated releases of several of
+the code in its repositories. These releases are generated by using 
+[Bitergia/release-tools](https://github.com/Bitergia/release-tools).
+
+### Changelog Entries
+
+The contributors need to create note entries about the changes in the code. 
+These notes will be included in the release notes. The contributors can use the 
+interactive [changelog](https://github.com/Bitergia/release-tools#changelog) 
+tool for this purpose. 
+
+Changelog entries use [YAML format](https://yaml.org/). Remember you can write blocks 
+of text using `>` character at the beginning of each block. See the next example:
+```
+title: 'Fix bug #666'
+category: fixed
+author: John Smith <jsmith@example.com>
+issue: 666
+notes: >
+  The bug was making impossible to cast a spell on
+  a magician.
+```
+
+These changelog entries would be included in the release notes which is read by 
+users who might not know anything about the internals of the code. The `notes` 
+should be written in simple language that is understandable by the audience.