add docs

Author: aviramha

README.md

@@ -176,6 +176,10 @@ the system's version of librdkafka. Example:
 rdkafka = { version = "0.25", features = ["dynamic-linking"] }
 ```
 
+If you'd like to compile librdkafka statically yourself, then use
+that, you can use `static-linking` while supplying `DEP_LIBRDKAFKA_STATIC_ROOT`
+with path to where librdkafka was built.
+
 For a full listing of features, consult the [rdkafka-sys crate's
 documentation][rdkafka-sys-features]. All of rdkafka-sys features are
 re-exported as rdkafka features.

rdkafka-sys/README.md

@@ -45,6 +45,11 @@ system, and it will configure the compiler to dynamically link against it.
 The system version of librdkafka must exactly match the version of
 librdkafka bundled with this crate.
 
+The **`static-linking`** feature can be used to link rdkafka to a locally
+built version of librdkafka: if the feature is enabled, the build script
+will try to find `DEP_LIBRDKAFKA_STATIC_ROOT` environment variable
+and it will statically link against it.
+
 The **`cmake-build`** feature builds librdkafka with its [CMake] build
 system, rather than its default [mklove]-based build system. This feature
 requires that CMake is installed on the build machine.