Improve documentation for issues, PR, change log

.github/ISSUE_TEMPLATE

@@ -1,4 +1,9 @@
-If you are posting about a *bug* please close this issue and open a new one
-using [SPC h I] or [M-m h I] (I = capital i) from within Spacemacs. If the
-bug makes Spacemacs unusable, you can find the bug report template [here](https://github.com/syl20bnr/spacemacs/blob/develop/core/templates/REPORTING.template)
+If you are posting about a *bug* please close this issue and open a new one from within Spacemacs using:
+- `SPC h I`
+- `M-m h I`
+
+(I = capital i)
+
+If the bug makes Spacemacs unusable, you can find the bug report template [here](https://github.com/syl20bnr/spacemacs/blob/develop/core/templates/REPORTING.template).
+
 You can delete this message to start typing.

.github/PULL_REQUEST_TEMPLATE

@@ -1,7 +1,7 @@
-Thank you for contributing to Spacemacs!
+Thank you :heart: for contributing to Spacemacs!
 
-Before you submit this pull request, please ensure it is against the develop branch and not master.
+Before you submit this pull request, please ensure it is against the `develop` branch and that you have read the [CONTRIBUTING.org](https://github.com/syl20bnr/spacemacs/blob/develop/CONTRIBUTING.org) file. It contains instructions on how to properly follow our conventions on commit messages, documentation, change log entries and other elements.
 
 This message should be replaced with a description of your change.
 
-Thank you <3
+Cheers!

CHANGELOG.develop

@@ -1,7 +1,36 @@
--*- mode: org -*-
+#+COMMENT -*- mode: org -*-
 
 This file containes the change log for the next major version of Spacemacs.
 
+Use the following template structure and fill it in with your changes.
+When a release will be drafted, the notes will be copied at the top of
+the [[file:CHANGELOG.org][CHANGELOG.org]] file.
+
+#+BEGIN_EXAMPLE
+* Release 0.400.x
+** 0.400.0
+*** Breaking Changes
+**** Major
+**** Key bindings
+**** Layers
+***** Layer 1
+***** Layer 2
+***** Layer 3
+*** Hot new features
+*** Dotfile changes
+*** Core changes
+*** Distribution changes
+*** Layers
+**** New layers
+**** Layer 1
+**** Layer 2
+**** Layer 3
+*** Various improvements
+**** Code syle and organization
+**** Documentation and website
+**** Testing and CI
+#+END_EXAMPLE
+
 * Release 0.300.x
 ** 0.300.0
 *** Breaking Changes