Update GitHub pull request template
Update GitHub pull request template

Add note about changelog description in template.
Description is supposed to be used to create changelog template.
See https://github.com/intellij-rust/intellij-rust.github.io/pull/98

Undin/intellij-rust,intellij-rust/intellij-rust,Undin/intellij-rust,Undin/intellij-rust,intellij-rust/intellij-rust,Undin/intellij-rust,intellij-rust/intellij-rust,Undin/intellij-rust,intellij-rust/intellij-rust,intellij-rust/intellij-rust,intellij-rust/intellij-rust,Undin/intellij-rust