Add release notes in Markdown format [skip ci]

This proposes to use Git to maintain notes about all notable changes to
GIL in `RELEASES.md` file based on https://keepachangelog.com format.
This implements proposal discussed and agreed in the thread
https://lists.boost.org/boost-gil/2019/11/0336.php

Collect all past release notes.
Collect release notes for the pending Boost 1.72.0, in section [Unreleased].

RELEASES.md

@@ -0,0 +1,62 @@
+# Release Notes
+
+All notable changes to [Boost.GIL](https://github.com/boostorg/gil/) project will be documented in this file.
+The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/).
+
+## [Unreleased]
+
+### Added
+- GSoC 2019: Lanczos resampling for image down scaling ([PR #309](https://github.com/boostorg/gil/pull/309)).
+- GSoC 2019: Methods for binary thresholding, inverted binary thresholding and truncation thresholding ([PR #313](https://github.com/boostorg/gil/pull/313)).
+- GSoC 2019: Otsu thresholding method ([PR #314](https://github.com/boostorg/gil/pull/314)).
+- GSoC 2019: Adaptive thresholding using mean or gaussian-weighted sum of the neighbourhood area ([PR #315](https://github.com/boostorg/gil/pull/315)).
+- GSoC 2019: Harris response calculation (corner detector without non-maximum filtering) ([PR #350](https://github.com/boostorg/gil/pull/350)).
+- GSoC 2019: Hessian corner detector ([PR #364](https://github.com/boostorg/gil/pull/364)).
+- GSoC 2019: Types for defining 2D kernel, `kernel_2d` and `kernel_2d_fixed`, in Numeric extension ([PR #361](https://github.com/boostorg/gil/pull/361)).
+- GSoC 2019: Implementation of 2D convolution as new function `convolve_2d` ([PR #367](https://github.com/boostorg/gil/pull/367)).
+- GSoC 2019: Box filtering using the average filter ([PR #383](https://github.com/boostorg/gil/pull/383)).
+- GSoC 2019: Blur function based on normalized mean filter ([PR #383](https://github.com/boostorg/gil/pull/383)).
+- GSoC 2019: Sobel and Scharr operators ([PR #392](https://github.com/boostorg/gil/pull/392)).
+- GSoC 2019: Median filter to remove noise from image ([PR #393](https://github.com/boostorg/gil/pull/393)).
+- Function `extend_boundary` in Numeric extension to perform image boundary extension ([PR #386](https://github.com/boostorg/gil/pull/386)).
+- Function `convolve_1d` in Numeric extension for convenient use of `convolve_rows` and `convolve_cols` ([PR #347](https://github.com/boostorg/gil/pull/347) and [PR #367](https://github.com/boostorg/gil/pull/367)).
+- Documented purpose of `cached_location_t` ([PR #287](https://github.com/boostorg/gil/pull/287)).
+- Continued adding new test cases and significantly improved overall test coverage.
+
+### Changed
+- Move all tests, core features and extensions, inside `test/` directory ([PR #302](https://github.com/boostorg/gil/pull/302)).
+
+### Removed
+- Dropped support for GCC <= 4.8 ([PR #296](https://github.com/boostorg/gil/pull/296)).
+- Replace Boost.MPL with Boost.MP11 ([PR #274](https://github.com/boostorg/gil/pull/274)).
+- Removed use of Boost.TypeTraits ([PR #274](https://github.com/boostorg/gil/pull/274)).
+
+### Fixed
+- Undetermined value of default-initialized channel and pixel objects ([PR #273](https://github.com/boostorg/gil/pull/273)).
+- Undefined behaviour due to `std::is_trivially_default_constructible` specializations ([PR #284](https://github.com/boostorg/gil/pull/284)).
+- Crash when reading PNG files with an invalid header ([PR #385](https://github.com/boostorg/gil/pull/385)).
+- Applied the [Rule of Three](https://en.wikipedia.org/wiki/Rule_of_three_(C%2B%2B_programming)) for numerous types.
+- Removed uses of deprecated implicit definition of defaulted copy assignment operator or copy constructor.
+
+## [1.68.0] - 2018-08-09
+
+### Added
+- The library now requires a C++11-compliant compiler.
+- Added Toolbox extension following the [review and acceptance into Boost](https://lists.boost.org/boost-announce/2011/01/0281.php).
+
+### Changed
+- The I/O extensions have been entirely rewritten as I/O v2, [reviewed and accepted into Boost](https://lists.boost.org/boost-announce/2011/01/0281.php).
+- Documentation has been reformatted and updated.
+
+### Removed
+- The existing I/O v1 extension has been replaced with I/O v2.
+
+## [1.53.0] - 2013-02-04
+
+### Fixed
+- Fixed self-assignment warnings (Trac [#4919](https://svn.boost.org/trac10/ticket/4919)).
+
+## [1.35.0] - 2008-03-29
+
+### Added
+- First release of Generic Image Library developed by Lubomir Bourdev and Hailin Jin following the [review and acceptance into Boost](https://lists.boost.org/Archives/boost/2006/11/112896.php).