Add CHANGELOG.md

Closes #356. Closes #352.

CHANGELOG.md

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+# Changelog
+
+All notable changes to this project will be documented in this file.
+
+The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.1.0/),
+and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0.html).
+
+## [Unreleased]
+
+_No unreleased changes_
+
+## [1.0.0] - 2020-01-07
+
+This is a major change, where modules are not git submodules in the Zim repo
+anymore, but customized and installed separately as individual repositories.
+External modules can more easily be installed, updated and uninstalled. This
+makes Zim the first project for Zsh that is both a set of community-maintained
+modules with a default installation (like on-my-zsh and prezto) and a plugin
+manager (like antigen and zplug).
+
+This version is not backwards-compatible with previous versions, so a new
+installation of Zim is required.
+
+Take your time to review the updated [README.md] and the changes listed below.
+
+### Added
+- `zimfw` CLI tool.
+- `zmodule` function to define modules.
+- Automatic installation script.
+- zsh-users/zsh-autosuggestions is enabled by default in new installations.
+
+### Changed
+- The Zim "core" is reduced to a single file, namely zimfw.zsh, that is
+  self-updated without requiring git. With this, `ZIM_HOME` is not (the root of)
+  a git repo anymore.
+- Zsh and modules are configured in .zshrc instead of .zimrc.
+- .zimrc is not sourced during Zsh startup anymore, and only contains the module
+  definitions.
+- Zim's init.zsh and login_init.zsh scripts are generated by the `zimfw` CLI
+  tool and contain static code. This allows for constant startup time,
+  regardless of how complex the module definitions are.
+- Zim modules moved to individual repositories in the https://github.com/zimfw
+  organization.
+- Zim modules are configured with `zstyle` instead of environment variables.
+- Zim themes are sourced directly, instead of working with prompinit, and are
+  configured with environment variables instead of with promptinit parameters.
+- The [environment] module is reduced in scope to to only set Zsh options. The
+  additional code moved to the [input] module (smart-URL widgets), the [utility]
+  module (default pager), and a new [termtitle] module (terminal window title).
+- The minimal theme is renamed to [s1ck94].
+
+### Removed
+- `zmanage` CLI tool.
+- The directory and history modules. Their code moved into the [environment]
+  module.
+- The prompt module, and the external lean, liquidprompt and pure themes.
+  Use `zmodule miekg/lean`, or `zmodule nojhan/liquidprompt`, or `zmodule
+  sindresorhus/pure --source async.zsh --source pure.zsh` to define one of these
+  external themes, respectively. The Zim themes moved to individual repositories.
+- Support for themes that require promptinit.
+  (See [#325](https://github.com/zimfw/zimfw/issues/325))
+
+### Fixed
+- `ZIM_HOME` is set in .zshenv instead of .zshrc. The issue was that the
+  variable was not available in .zlogin in non-interactive login shells.
+
+[README.md]: https://github.com/zimfw/zimfw/blob/master/README.md
+[environment]: https://github.com/zimfw/environment
+[input]: https://github.com/zimfw/input
+[utility]: https://github.com/zimfw/utility
+[termtitle]: https://github.com/zimfw/termtitle
+[s1ck94]: https://github.com/zimfw/s1ck94
+
+[Unreleased]: https://github.com/zimfw/zimfw/compare/v1.0.0...HEAD
+[1.0.0]: https://github.com/zimfw/zimfw/compare/5d66578...v1.0.0