Update readme in topics and subtopics to improve readability.

readme.md

@@ -1,10 +1,25 @@
 # Important Links
 
-- [gradeimporter Repository](http://200.144.254.107/git/bernardo/gradeImporter)
+## Repositories
+
+- [gradeimporter](http://200.144.254.107/git/bernardo/gradeImporter)
+- [iassign - working activity plugin developed by LInE](http://200.144.254.107/git/LInE/iassign)
+
+## Moodle
+
 - [Moodle School](https://moodledev.moodle.school/ )
 - [Repository Plugin - MoodleDocs](https://docs.moodle.org/dev/Repository_plugins)
+- [New module documentation](https://docs.moodle.org/dev/Tutorial)
 - [Introduction to Activity modules](https://docs.moodle.org/dev/Activity_modules)
-- [String API](https://docs.moodle.org/dev/String_API)
-- [new module documentation](https://docs.moodle.org/dev/Tutorial)
+
+### Support plugins
+
 - [XMLDB - plugin to help build install.xml](https://docs.moodle.org/dev/Using_XMLDB)
-- [iassign repository - working activity plugin developed by LInE](http://200.144.254.107/git/LInE/iassign)
+- [PluginSkel](https://moodle.org/plugins/tool_pluginskel)
+
+### APIs
+
+- [String API](https://docs.moodle.org/dev/String_API)
+- [File API](https://docs.moodle.org/dev/File_API)
+  - [Files in Forms](https://docs.moodle.org/dev/Using_the_File_API_in_Moodle_forms)
+  - [File API internals](https://docs.moodle.org/dev/File_API_internals)