adds README and screenshot

README.md

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+# Explain — A Liquid Prompt Theme
+
+*Explain* is a theme for [Liquid Prompt](https://github.com/nojhan/liquidprompt).
+
+It aims at showing the meaning of each section of the default prompt theme.
+To do so it displays:
+
+- a box around each section,
+- with the name of the corresponding variable in the default theme's template,
+- and it adds an hyperlink to it, so that the user can click and rapidly jump to the documentation.
+
+
+![image](explain-demo.png)
+
+
+## Configuration
+
+### Color of the explanation
+
+**LP_COLOR_EXPLAIN** *color* = 239 -1 (dark grey)
+
+Color for everything surrounding the actual prompt sections.
+
+### Liquid Prompt Configuration
+
+**LP_MARK_PREFIX** *string* = $'\n'
+
+The prefix of the duplicated prompt mark.
+
+### Disabled Configuration
+
+The following variables are disabled:
+
+- LP_MARK_DEV_OPEN
+- LP_MARK_DEV_CLOSE
+- LP_MARK_DEV_MID
+- LP_MARK_MODULES_OPEN
+- LP_MARK_MODULES_SEP
+- LP_MARK_MODULES_CLOSE
+- LP_MARK_ENV_VARS_OPEN
+- LP_MARK_ENV_VARS_SEP
+- LP_MARK_ENV_VARS_CLOSE
+