Small cleanup of PR and issue templates

- Add links to PULL_REQUEST_TEMPLATE.md
- Clean `System information` table

It's easier to find needes documentation about
PR process with links.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Giuseppe Di Natale <guss80@gmail.com>
Signed-off-by: George Melikov <mail@gmelikov.ru>
Closes #7539

.github/ISSUE_TEMPLATE.md

@@ -14,7 +14,7 @@ Please fill in as much of the template as possible.
 -->
 
 ### System information
-<!--  add version after "|" character -->
+<!--  add version after "|" character -->
 Type | Version/Name
  --- | --- 
 Distribution Name	| 

.github/PULL_REQUEST_TEMPLATE.md

@@ -30,10 +30,10 @@ https://github.com/zfsonlinux/zfs/wiki/Buildbot-Options
 ### Checklist:
 <!--- Go over all the following points, and put an `x` in all the boxes that apply. -->
 <!--- If you're unsure about any of these, don't hesitate to ask. We're here to help! -->
-- [ ] My code follows the ZFS on Linux code style requirements.
+- [ ] My code follows the ZFS on Linux [code style requirements](https://github.com/zfsonlinux/zfs/blob/master/.github/CONTRIBUTING.md#coding-conventions).
 - [ ] I have updated the documentation accordingly.
-- [ ] I have read the **CONTRIBUTING** document.
-- [ ] I have added tests to cover my changes.
+- [ ] I have read the [**contributing** document](https://github.com/zfsonlinux/zfs/blob/master/.github/CONTRIBUTING.md).
+- [ ] I have added [tests](https://github.com/zfsonlinux/zfs/tree/master/tests) to cover my changes.
 - [ ] All new and existing tests passed.
-- [ ] All commit messages are properly formatted and contain `Signed-off-by`.
+- [ ] All commit messages are properly formatted and contain [`Signed-off-by`](https://github.com/zfsonlinux/zfs/blob/master/.github/CONTRIBUTING.md#signed-off-by).
 - [ ] Change has been approved by a ZFS on Linux member.