Be uniform in heading spacing

We currently sometimes have one newline before headings and sometimes
two, its not important which it is but uniformity is nice.

Use two newlines before headings uniformly in `CONTRIBUTING.md`.

Author: tcharding

CONTRIBUTING.md

@@ -8,6 +8,7 @@ implementation and other Rust Bitcoin-related projects, which are hosted in the
 mostly guidelines, not rules. Use your best judgment, and feel free to propose
 changes to this document in a pull request.
 
+
 #### Table Of Contents
 
 - [General](#general)
@@ -97,6 +98,7 @@ comment suggesting that you're working on it. If someone is already assigned,
 don't hesitate to ask if the assigned party or previous commenters are still
 working on it if it has been awhile.
 
+
 ## Preparing PRs
 
 The main library development happens in the `master` branch. This branch must
@@ -136,6 +138,7 @@ NB: Please keep in mind that the script above replaces `Cargo.lock` file, which
 is necessary to support current MSRV, incompatible with `stable` and newer cargo
 versions.
 
+
 ### Peer review
 
 Anyone may participate in peer review which is expressed by comments in the pull
@@ -144,6 +147,7 @@ test out the patch set and opine on the technical merits of the patch. Please,
 first review PR on the conceptual level before focusing on code style or
 grammar fixes.
 
+
 ### Repository maintainers
 
 Pull request merge requirements:
@@ -169,6 +173,7 @@ Current list of the project maintainers:
 
 Library reflects Bitcoin Core approach whenever possible.
 
+
 ### Formatting
 
 The repository currently does not use `rustfmt`.
@@ -184,12 +189,14 @@ and [how it is planned to coordinate it with crate refactoring](https://github.c
 For the new code it is recommended to follow style of the existing codebase and
 avoid any end-line space characters.
 
+
 ### MSRV
 
 The Minimal Supported Rust Version (MSRV) is 1.29; it is enforced by our CI.
 Later we plan to increase MSRV to support Rust 2018 and you are welcome to check
 the [tracking issue](https://github.com/rust-bitcoin/rust-bitcoin/issues/510).
 
+
 ### Naming conventions
 
 Naming of data structures/enums and their fields/variants must follow names used
@@ -198,6 +205,7 @@ in Bitcoin Core, with the following exceptions:
   snake_case for fields and variants);
 - omit `C`-prefixes.
 
+
 ### Unsafe code
 
 Use of `unsafe` code is prohibited unless there is a unanimous decision among