Add Readme to pj plugin

plugins/pj/README.md

+# pj
+
+The `pj` plugin (short for `Project Jump`) allows you to define several
+folders where you store your projects, so that you can jump there directly
+by just using the name of the project directory.
+
+Original idea and code by Jan De Poorter ([@DefV](https://github.com/DefV))
+Source: https://gist.github.com/pjaspers/368394#gistcomment-1016
+
+## Usage
+
+1. Enable the `pj` plugin:
+
+   ```zsh
+   plugins=(... pj)
+   ```
+
+2. Set `$PROJECT_PATHS` in your ~/.zshrc:
+
+   ```zsh
+   PROJECT_PATHS=(~/src ~/work ~/"dir with spaces")
+   ```
+
+You can now use one of the following commands:
+
+##### `pj my-project`:
+
+`cd` to the directory named "my-project" found in one of the `$PROJECT_PATHS`
+directories. If there are several directories named the same, the first one
+to appear in `$PROJECT_PATHS` has preference.
+
+For example:
+```zsh
+PROJECT_PATHS=(~/code ~/work)
+$ ls ~/code    # ~/code/blog ~/code/react
+$ ls ~/work    # ~/work/blog ~/work/project
+$ pj blog      # <-- will cd to ~/code/blog
+```
+
+##### `pjo my-project`
+
+Open the project directory with your defined `$EDITOR`. This follows the same
+directory rules as the `pj` command above.
+
+Note: `pjo` is an alias of `pj open`.