doc: move CHANGELOG from doc/ to root directory

Author: jonasnick

doc/CHANGELOG.md CHANGELOG.md

@@ -1,7 +1,7 @@
 # Release Process
 
 1. Open PR to master that
-   1. adds release notes to `doc/CHANGELOG.md` and
+   1. adds release notes to `CHANGELOG.md` and
    2. if this is **not** a patch release, updates `_PKG_VERSION_{MAJOR,MINOR}` and `_LIB_VERSIONS_*` in `configure.ac`
 2. After the PR is merged,
    * if this is **not** a patch release, create a release branch with name `MAJOR.MINOR`.
@@ -11,4 +11,4 @@
      Also include the release note commit bump `_PKG_VERSION_BUILD` and `_LIB_VERSIONS_*` in `configure.ac`.
 4. Tag the commit with `git tag -s vMAJOR.MINOR.PATCH`.
 5. Push branch and tag with `git push origin --tags`.
-6. Create a new GitHub release with a link to the corresponding entry in `doc/CHANGELOG.md`.
+6. Create a new GitHub release with a link to the corresponding entry in `CHANGELOG.md`.