docs(readme): document segmented parallel download API

Author: FarnaHerry

README.md

@@ -7,6 +7,7 @@ Minimal C++23 HTTP/HTTPS client library with SSE (Server-Sent Events) streaming
 - HTTP/HTTPS client with connection pooling (keep-alive)
 - SSE (Server-Sent Events) streaming
 - Proxy support (HTTP CONNECT)
+- Segmented parallel downloads via HTTP Range (aria2-style)
 - C++23 modules
 
 ## Usage
@@ -28,6 +29,41 @@ auto resp = client.send(mcpplibs::tinyhttps::HttpRequest::post(
 ));
 ```
 
+## Parallel downloads
+
+`download_to_file_parallel()` probes the server with `Range: bytes=0-0`; when
+the server answers 206 the file is split into segments fetched concurrently
+into a pre-allocated file. It falls back to a plain sequential download when
+the server ignores Range (200) or the file is too small to split.
+
+```cpp
+import mcpplibs.tinyhttps;
+namespace https = mcpplibs::tinyhttps;
+
+https::HttpClientConfig cfg;
+cfg.maxConnectionsPerFile = 8;          // concurrent segment workers
+cfg.maxSegments         = 16;           // aria2 -s: split count (0 = tie to connections)
+cfg.minSegmentBytes     = 4 << 20;      // aria2 --min-split-size: 4 MiB
+
+https::HttpClient client(cfg);
+auto result = client.download_to_file_parallel(
+    "https://example.com/big.iso",
+    "big.iso",
+    [](std::int64_t total, std::int64_t done) {
+        // monotonic progress; total is 0 when unknown
+    },
+    [] { return false; }                // return true to cancel
+);
+if (result.ok()) { /* result.bytesWritten, result.expectedBytes, ... */ }
+```
+
+Behavior notes:
+
+- Segment boundaries never overlap; interrupted segments resume mid-range on
+  retry (up to 2 retries per segment).
+- Progress callbacks are serialized and monotonically increasing.
+- The pre-allocated target file is removed if the download fails.
+
 ## 使用 mcpp 构建
 
 ### 添加依赖