git-prompt: add README (#7554)

plugins/git-prompt/README.md

+# git-prompt plugin
+
+A `zsh` prompt that displays information about the current git repository. In particular:
+the branch name, difference with remote branch, number of files staged or changed, etc.
+
+To use it, add `git-prompt` to the plugins array in your zshrc file:
+
+```zsh
+plugins=(... git-prompt)
+```
+
+See the [original repository](https://github.com/olivierverdier/zsh-git-prompt).
+
+## Examples
+
+The prompt may look like the following:
+
+- `(master↑3|✚1)`: on branch `master`, ahead of remote by 3 commits, 1 file changed but not staged
+- `(status|●2)`: on branch `status`, 2 files staged
+- `(master|✚7…)`: on branch `master`, 7 files changed, some files untracked
+- `(master|✖2✚3)`: on branch `master`, 2 conflicts, 3 files changed
+- `(experimental↓2↑3|✔)`: on branch `experimental`; your branch has diverged by 3 commits, remote by 2 commits; the repository is otherwise clean
+- `(:70c2952|✔)`: not on any branch; parent commit has hash `70c2952`; the repository is otherwise clean
+
+## Prompt Structure
+
+By default, the general appearance of the prompt is:
+
+```
+(<branch><branch tracking>|<local status>)
+```
+
+The symbols are as follows:
+
+### Local Status Symbols
+
+| Symbol | Meaning                        |
+|--------|--------------------------------|
+| ✔      | repository clean               |
+| ●n     | there are `n` staged files     |
+| ✖n     | there are `n` unmerged files   |
+| ✚n     | there are `n` unstaged files   |
+| …      | there are some untracked files |
+
+### Branch Tracking Symbols
+
+| Symbol | Meaning                                                       |
+|--------|---------------------------------------------------------------|
+| ↑n     | ahead of remote by `n` commits                                |
+| ↓n     | behind remote by `n` commits                                  |
+| ↓m↑n   | branches diverged: other by `m` commits, yours by `n` commits |
+
+## Customisation
+
+- Set the variable `ZSH_THEME_GIT_PROMPT_CACHE` to any value in order to enable caching.
+- You may also change a number of variables (whose name start with `ZSH_THEME_GIT_PROMPT_`) 
+  to change the appearance of the prompt. Take a look at the bottom of the [plugin file](git-prompt.plugin.zsh)`
+  to see what variables are available.
+
+
+**Enjoy!**

plugins/git-prompt/git-prompt.plugin.zsh

-# ZSH Git Prompt Plugin from:
-# https://github.com/olivierverdier/zsh-git-prompt
-
 __GIT_PROMPT_DIR="${0:A:h}"
 
 ## Hook function definitions