README.md: Put some requirements on installation instructions

Adding new installation methods increases risk of supply chain exploits
and increases burden of maintainers. Let's put some requirements on what
installation methods to have in the README.md

README.md

@@ -265,6 +265,15 @@ Please report any issues with the help syntax in [this repository](https://githu
 
 ## Installation
 
+<!--
+
+Installation instructions need to:
+* be for widely used systems
+* be non-obvious
+* be from somewhat official sources
+
+-->
+
 [![Packaging status](https://repology.org/badge/vertical-allrepos/bat-cat.svg?columns=3&exclude_unsupported=1)](https://repology.org/project/bat-cat/versions)
 
 ### On Ubuntu (using `apt`)
@@ -327,14 +336,6 @@ You can install [the `bat` package](https://koji.fedoraproject.org/koji/packagei
 dnf install bat
 ```
 
-### On Funtoo Linux
-
-You can install [the `bat` package](https://github.com/funtoo/dev-kit/tree/1.4-release/sys-apps/bat) from dev-kit.
-
-```bash
-emerge sys-apps/bat
-```
-
 ### On Gentoo Linux
 
 You can install [the `bat` package](https://packages.gentoo.org/packages/sys-apps/bat)
@@ -344,20 +345,6 @@ from the official sources:
 emerge sys-apps/bat
 ```
 
-### On Void Linux
-
-You can install `bat` via xbps-install:
-```bash
-xbps-install -S bat
-```
-
-### On Termux
-
-You can install `bat` via pkg:
-```bash
-pkg install bat
-```
-
 ### On FreeBSD
 
 You can install a precompiled [`bat` package](https://www.freshports.org/textproc/bat) with pkg:
@@ -389,14 +376,6 @@ You can install `bat` using the [nix package manager](https://nixos.org/nix):
 nix-env -i bat
 ```
 
-### Via flox
-
-You can install `bat` using [Flox](https://flox.dev)
-
-```bash
-flox install bat
-```
-
 ### On openSUSE
 
 You can install `bat` with zypper: