README: Add Gentoo documentation

binutils-libs and cmake are already build time dependency for clang
but won't get pulled in when using the binary package as they don't
need build time dependencies so add them to the set too.

Signed-off-by: zyxhere💭 <zyx@envs.net>

Author: zyxhere

README.md

@@ -89,6 +89,45 @@ These scripts have been tested in a Docker image of the following distributions
             python3 \
             uboot-tools
   ```
+* ### Gentoo
+
+  A convenient way to manage all the required packages is to use [package sets](https://wiki.gentoo.org/wiki//etc/portage/sets). Create a set for tc-build:
+
+  ```
+  $ sudo mkdir /etc/portage/sets && sudo $EDITOR /etc/portage/sets/tc-build
+  ```
+
+  With the file `tc-build` containing:
+
+  ```
+  llvm-core/clang
+  llvm-core/lld
+  sys-libs/binutils-libs
+  dev-build/cmake
+  dev-vcs/git
+  dev-util/ccache
+  dev-libs/elfutils
+  app-arch/cpio
+  dev-embedded/u-boot-tools
+  ```
+
+  Afterwards emerge the set:
+
+  ```
+  $ sudo emerge --ask --verbose @tc-build
+  ```
+
+  To avoid building from source Gentoo also provides [binary packages](https://wiki.gentoo.org/wiki/Binary_package_guide).
+
+  ```
+  $ sudo emerge --ask --verbose --getbinpkg @tc-build
+  ```
+
+  Or for short:
+
+  ```
+  $ sudo emerge -avg @tc-build
+  ```
 
 * ### Clear Linux