Merge pull request #4489 from facebook/doc_install

Added documentation details for Makefile installation and pkg-config.

Author: Cyan4973

README.md

@@ -127,19 +127,19 @@ When your system allows it, prefer using `make` to build `zstd` and `libzstd`.
 
 ### Makefile
 
-If your system is compatible with standard `make` (or `gmake`),
+Assuming your system supports standard `make` (or `gmake`),
 invoking `make` in root directory will generate `zstd` cli in root directory.
 It will also create `libzstd` into `lib/`.
 
-Other available options include:
+Other standard targets include:
 - `make install` : create and install zstd cli, library and man pages
 - `make check` : create and run `zstd`, test its behavior on local platform
 
 The `Makefile` follows the [GNU Standard Makefile conventions](https://www.gnu.org/prep/standards/html_node/Makefile-Conventions.html),
-allowing staged install, standard flags, directory variables and command variables.
+allowing staged install, standard compilation flags, directory variables and command variables.
 
-For advanced use cases, specialized compilation flags which control binary generation
-are documented in [`lib/README.md`](lib/README.md#modular-build) for the `libzstd` library
+For advanced use cases, specialized flags which control binary generation and installation paths are documented
+in [`lib/README.md`](lib/README.md#modular-build) for the `libzstd` library
 and in [`programs/README.md`](programs/README.md#compilation-variables) for the `zstd` CLI.
 
 ### cmake

lib/README.md

@@ -7,15 +7,29 @@ in order to make it easier to select or exclude features.
 
 #### Building
 
-`Makefile` script is provided, supporting [Makefile conventions](https://www.gnu.org/prep/standards/html_node/Makefile-Conventions.html#Makefile-Conventions),
+A `Makefile` script is provided, supporting [Makefile conventions](https://www.gnu.org/prep/standards/html_node/Makefile-Conventions.html#Makefile-Conventions),
 including commands variables, staged install, directory variables and standard targets.
 - `make` : generates both static and dynamic libraries
-- `make install` : install libraries and headers in target system directories
+- `make install` : install libraries, headers and pkg-config in local system directories
 
-`libzstd` default scope is pretty large, including compression, decompression, dictionary builder,
-and support for decoding legacy formats >= v0.5.0.
+`libzstd` default scope is extensive, including compression, decompression, dictionary builder,
+and support for decoding legacy formats >= v0.5.0 by default.
 The scope can be reduced on demand (see paragraph _modular build_).
 
+#### Multiarch Support
+
+For multiarch systems (like Debian/Ubuntu), libraries should be installed to architecture-specific directories.
+When creating packages for such systems, use the `LIBDIR` variable to specify the correct multiarch path:
+
+```bash
+# For x86_64 systems on Ubuntu/Debian:
+make install PREFIX=/usr LIBDIR=/usr/lib/x86_64-linux-gnu
+
+# For ARM64 systems on Ubuntu/Debian:
+make install PREFIX=/usr LIBDIR=/usr/lib/aarch64-linux-gnu
+```
+
+This will not only install the files in the correct directories, but also generate the correct paths for `pkg-config`.
 
 #### Multithreading support