Improve pull_request_template

commit message is copied at the end of the template when creating PR,
so swap instructions and template part

Signed-off-by: Etienne CHAMPETIER <champetier.etienne@gmail.com>

.github/pull_request_template

@@ -1,10 +1,3 @@
-Maintainer: me / @<github-user>
-Compile tested: (put here arch, model, OpenWRT/LEDE version)
-Run tested: (put here arch, model, OpenWRT/LEDE version, tests done)
-
-Description:
-
--------------------------------
 Please double check that your commits:
 - all start with "<package name>: "
 - all contain signed-off-by
@@ -13,4 +6,11 @@ Please double check that your commits:
 Please also read https://github.com/openwrt/packages/blob/master/CONTRIBUTING.md
 
 Thanks for your contribution
-To show that you've read until here, please remove everything after ---
+Please remove this text (before ---) and fill the following template
+-------------------------------
+
+Maintainer: me / @<github-user>
+Compile tested: (put here arch, model, OpenWRT/LEDE version)
+Run tested: (put here arch, model, OpenWRT/LEDE version, tests done)
+
+Description: