changelog: add a changelog for the massive refactoring

CHANGELOG.md

+# v0.1210.1 (unreleased)
+
+## New API strategy
+
+A new pattern for API implementation is now underway. Instead of methods
+directly on the `Gitlab` instance, there are now structures which implement an
+`api::Endpoint` trait. This trait may be used to query any structure
+implementing the `api::Client` trait using the `api::Query` trait. All
+endpoints use the "builder" pattern to collect required and optional
+parameters.
+
+There are some adaptor functions to handle various use cases:
+
+  - `api::paged`: This may be used to handle pagination of any endpoint which
+    supports it (checked at compile time).
+  - `api::ignore`: This may be used to ignore the content of the response for
+    any endpoint. HTTP and GitLab error messages are still captured.
+  - `api::raw`: Instead of deserializing the contents of the result from GitLab
+    into a structure, the raw bytes may be fetched instead using this function.
+  - `api::sudo`: This function adapts any endpoint into being called as another
+    user if the client is able to do so (basically, is an administrator).
+
+The `api::Query` trait deserializes the contents from GitLab into any structure
+which implements the `serde::DeserializeOwned` trait. This can be used to only
+grab information of interest to the caller instead of extracting all of the
+information available through the `types` module.
+
+If your endpoint is deprecated, it has been marked as such and you should
+migrate to the new pattern. Please see the docs for available endpoints.
+
+All new endpoint implementations should use the new pattern rather than adding
+methods to `Gitlab`. Result structures do not need to be added to this crate
+either. It is expected that they too will be deprecated at some point and
+either not provided or moved to a dedicated crate.
+
+### Examples:
+
+```rust
+use std::env;
+
+use serde::Deserialize;
+use gitlab::Gitlab;
+use gitlab::api::{self, projects, Query};
+
+#[derive(Debug, Deserialize)]
+struct Project {
+    name: String,
+}
+
+fn example() {
+    // Create the client.
+    let client = Gitlab::new("gitlab.com", env::get("GITLAB_TOKEN").unwrap()).unwrap();
+
+    // Create a simple endpoint.
+    let endpoint = projects::Project::builder().project("gitlab-org/gitlab").build().unwrap();
+    // Get the information.
+    let project: Project = endpoint.query(&client).unwrap();
+    // Call it again, but ignore the response from GitLab.
+    let _: () = api::ignore(endpoint).query(&client).unwrap();
+
+    // Create an endpoint that supports pagination.
+    let pageable_endpoint = projects::Projects::builder().build().unwrap();
+    // Get just the first page (20 results).
+    let first_page: Vec<Project> = pageable_endpoint.query(&client).unwrap();
+    // Get 200 results instead.
+    let first_200_projects: Vec<Project> = api::paged(pageable_endpoint, api::Pagination::Limit(200)).query(&client).unwrap();
+
+    // Query `gitlab-org/gitlab` except by ID this time.
+    let endpoint = projects::Project::builder().project(278964).build().unwrap();
+    // Get the raw data from the response.
+    let raw_data: Vec<u8> = api::raw(endpoint).query(&client).unwrap();
+}
+```
+
+## Changes
+
+  * Include a changelog.