doc: Add ABI checking with check-abi.sh to the Release Process

doc/release-process.md

@@ -37,7 +37,12 @@ gcc -o ecdsa examples/ecdsa.c -I $dir/include -L $dir/lib*/ -l secp256k1 -Wl,-rp
 
 ## Regular release
 
-1. Open a PR to the master branch with a commit (using message `"release: prepare for $MAJOR.$MINOR.$PATCH"`, for example) that
+1. Check the ABI compatibility with the [`check-abi.sh`](/tools/check-abi.sh) tool (installing the `abi-dumper` and `abi-compliance-checker` packages might be required):
+```shell
+tools/check-abi.sh $(git describe --abbrev=0) master
+```
+
+2. Open a PR to the master branch with a commit (using message `"release: prepare for $MAJOR.$MINOR.$PATCH"`, for example) that
    * finalizes the release notes in [CHANGELOG.md](../CHANGELOG.md) by
        * adding a section for the release (make sure that the version number is a link to a diff between the previous and new version),
        * removing the `[Unreleased]` section header, and
@@ -46,19 +51,20 @@ gcc -o ecdsa examples/ecdsa.c -I $dir/include -L $dir/lib*/ -l secp256k1 -Wl,-rp
    * if this is not a patch release
        * updates `_PKG_VERSION_*` and `_LIB_VERSION_*`  in `configure.ac` and
        * updates `project(libsecp256k1 VERSION ...)` and `${PROJECT_NAME}_LIB_VERSION_*` in `CMakeLists.txt`.
-2. After the PR is merged, tag the commit and push it:
+
+3. After the PR is merged, tag the commit and push it:
    ```
    RELEASE_COMMIT=<merge commit of step 1>
    git tag -s v$MAJOR.$MINOR.$PATCH -m "libsecp256k1 $MAJOR.$MINOR.$PATCH" $RELEASE_COMMIT
    git push [email protected]:bitcoin-core/secp256k1.git v$MAJOR.$MINOR.$PATCH
    ```
-3. Open a PR to the master branch with a commit (using message `"release cleanup: bump version after $MAJOR.$MINOR.$PATCH"`, for example) that
+4. Open a PR to the master branch with a commit (using message `"release cleanup: bump version after $MAJOR.$MINOR.$PATCH"`, for example) that
    * sets `_PKG_VERSION_IS_RELEASE` to `false` and increments `_PKG_VERSION_PATCH` and `_LIB_VERSION_REVISION` in `configure.ac`,
    * increments the `$PATCH` component of `project(libsecp256k1 VERSION ...)` and `${PROJECT_NAME}_LIB_VERSION_REVISION` in `CMakeLists.txt`, and
    * adds an `[Unreleased]` section header to the [CHANGELOG.md](../CHANGELOG.md).
 
    If other maintainers are not present to approve the PR, it can be merged without ACKs.
-4. Create a new GitHub release with a link to the corresponding entry in [CHANGELOG.md](../CHANGELOG.md).
+5. Create a new GitHub release with a link to the corresponding entry in [CHANGELOG.md](../CHANGELOG.md).
 
 ## Maintenance release