[README] add notes for screen (256color/24bit) (#768)

Ref.: https://github.com/dandavison/delta/issues/760

1 files changed, 21 insertions, 0 deletions

diff --git a/README.md b/README.md
index 67fcdc23..ad775628 100644
--- a/README.md
+++ b/README.md
@@ -69,6 +69,7 @@ Code evolves, and we all spend time studying diffs. Delta aims to make this both
   - [--color-moved support](#--color-moved-support)
   - [Navigation keybindings for large diffs](#navigation-keybindings-for-large-diffs)
   - [24 bit color (truecolor)](#24-bit-color-truecolor)
+  - [Using Delta with GNU Screen](#using-delta-with-gnu-screen)
   - [Using Delta on Windows](#using-delta-on-windows)
   - [Mouse scrolling](#mouse-scrolling)
   - [Using Delta with Magit](#using-delta-with-magit)
@@ -499,6 +500,26 @@ set -ga terminal-overrides ",xterm-256color:Tc"
 
 and you may then need to quit tmux completely for it to take effect.
 
+True color output in GNU Screen is currently only possible when using a development build, as support for it is not yet implemented in the (v4) release versions. A snapshot of the latest Git trunk can be obtained via https://git.savannah.gnu.org/cgit/screen.git/snapshot/screen-master.tar.gz - the required build steps are described in the `src/INSTALL` file. After installing the program, 24-bit color support can be activated by including `truecolor on` in either the system's or the user's `screenrc` file.
+
+### Using Delta with GNU Screen
+
+When working in Screen without true color output, it might be that colors supposed to be different look the same in XTerm compatible terminals. If that is the case, make sure the following settings are included in your `screenrc` file:
+
+```cfg
+term screen-256color
+termcapinfo xterm 'Co#256:AB=\E[48;5;%dm:AF=\E[38;5;%dm'  # ANSI (256-color) patterns - AB: background, AF: foreground
+attrcolor b ".I"                                          # use bright colors for bold text
+```
+
+If despite having those settings you still only get a limited set of colors, your build of Screen might have been configured without the `--enable-colors256` flag. If this is the case, you have two options :
+
+- If available for your OS, get a different package of Screen. Otherwise
+- Build your own binary :
+    * Download and extract a release tarball from https://ftp.gnu.org/gnu/screen/
+    * `cd` into the newly extracted folder
+    * Follow the instructions in the `INSTALL` file, and when running the `./configure` command apply the `--enable-colors256` flag.
+
 ### Using Delta on Windows
 
 Delta works on Windows. However, it is essential to use a recent version of `less.exe`: you can download one from https://github.com/jftuga/less-Windows/releases/latest. If you see incorrect colors and/or strange characters in Delta output, then it is probably because Delta is picking up an old version of `less.exe` on your system.