Create README.md

README.md

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+# ESPTerm web interface
+
+This repository contains the ESPTerm web interface source files.
+
+## Installation
+
+Run `yarn install` (if a `yarn.lock` file is present) to set up the build tools 
+used for JS and SASS. (Otherwise, globally install `sass`.)
+
+You also need PHP installed (preferably version >= 7, but older may work too).
+PHP is used to build the HTML files and apply substitutions.
+
+## Development
+
+JavaScript source files can be found in the `jssrc/` folder, SASS
+files in the `sass` folder.
+
+Fontello (icon font) is maintained in the `fontello/` folder. To update Fontello, replace
+the `fontello.zip` file and run `unpack.sh` in the same folder. This will extract the needed CSS
+from the zip file and put them into the `sass/` folder to be included with the other styles.
+
+To test you changes (after building JS and CSS), run a PHP local server in the project
+directory using the `server.sh` script or by a command like `php -S 0.0.0.0:2000`.
+Template substitutions (that are normally done by the ESPTerm's webserver) applied to the 
+files fior testing can be defined in `_debug_replacements.php`.
+
+### Template strings
+
+Template strings are used to embed substitutions dynamically when serving the web page.
+They have the format `%key%`, `%j:key%` or `%h:key%`, the prefix `j:` and `h:`
+causes the value to be HTML- or JS-escaped.
+
+HTML escape turns `'"<>` into HTML entities. JS escape turns `'"\<>\r\n` into the 
+corresponding JS-string escape sequences (`<>` use `\u003C` and `\u003E`).
+
+## Building
+
+To build the final JS and CSS files, run `build.sh`. This script also puts static versions 
+of the HTML files in the `out/` folder to be retrieved by the ESPTerm's firmware build script.
+
+Files with template substitutions use the `.tpl` suffix. This ensures they are not pre-gzipped
+when building the ESPTerm firmware, as `.html` files are sent to the browser already compressed
+to save file size and substitutions would not be possible.
+
+### Demo build
+
+If the env variable ESP_DEMO is set when running `build.sh`, those static HTML
+files will be altered for use in the ESPTerm web demo (i.e. the JS will make
+no attempts to communicate with the back-end and use dummy values instead.
+
+All files use the `.html` suffix in a demo build and have replacements applied using 
+`_debug_replacements.php`.
+
+