Add installation instructions for Gentoo Linux to README.md (#1134)

* Add installation instructions for Gentoo Linux to README.md

* Correct installation instructions for macOS (nix) in README.md

* Move installation instructions for Gentoo Linux from README.md to docs/installation.rst

---------

Co-authored-by: Fonic <5cx5jbnmt@mozmail.com>

README.md

@@ -70,7 +70,7 @@ Linux, Windows, macOS and FreeBSD are supported. Docker images are also availabl
 | Windows Subsystem for Linux   | ``apt install ocrmypdf``      |
 | Fedora                        | ``dnf install ocrmypdf``      |
 | macOS (Homebrew)              | ``brew install ocrmypdf``     |
-| macOS (nix)                   | ``nix-env -i  ocrmypdf``      |
+| macOS (nix)                   | ``nix-env -i ocrmypdf``       |
 | LinuxBrew                     | ``brew install ocrmypdf``     |
 | FreeBSD                       | ``pkg install py-ocrmypdf``   |
 | Conda                         | ``conda install ocrmypdf``    |

docs/installation.rst

@@ -275,6 +275,21 @@ To install OCRmyPDF for Alpine Linux:
 
     apk add ocrmypdf
 
+Gentoo Linux
+------------
+
+.. image:: https://repology.org/badge/version-for-repo/gentoo_ovl_guru/ocrmypdf.svg
+    :alt: Gentoo Linux
+    :target: https://repology.org/metapackage/ocrmypdf
+
+To install OCRmyPDF on Gentoo Linux, use the following commands:
+
+.. code-block:: bash
+
+    eselect repository enable guru
+    emaint sync --repo guru
+    emerge --ask app-text/OCRmyPDF
+
 Other Linux packages
 --------------------