Add/polish the readme for all addons

Fixes #2193

addons/xterm-addon-attach/README.md

@@ -0,0 +1,22 @@
+## xterm-addon-attach
+
+An addon for [xterm.js](https://github.com/xtermjs/xterm.js) that enables attaching to a web socket. This addon requires xterm.js v4+.
+
+### Install
+
+```bash
+npm install --save xterm-addon-attach
+```
+
+### Usage
+
+```ts
+import { Terminal } from 'xterm';
+import { FitAddon } from 'xterm-addon-attach';
+
+const terminal = new Terminal();
+const attachAddon = new AttachAddon(webSocket);
+terminal.loadAddon(attachAddon);
+```
+
+See the full [API](https://github.com/xtermjs/xterm.js/blob/master/addons/xterm-addon-attach/typings/xterm-addon-attach.d.ts) for more advanced usage.

addons/xterm-addon-fit/README.md

@@ -0,0 +1,24 @@
+## xterm-addon-fit
+
+An addon for [xterm.js](https://github.com/xtermjs/xterm.js) that enables fitting the terminal's dimensions to a containing element. This addon requires xterm.js v4+.
+
+### Install
+
+```bash
+npm install --save xterm-addon-fit
+```
+
+### Usage
+
+```ts
+import { Terminal } from 'xterm';
+import { FitAddon } from 'xterm-addon-fit';
+
+const terminal = new Terminal();
+const fitAddon = new FitAddon();
+terminal.loadAddon(fitAddon);
+terminal.open(containerElement);
+fitAddon.fit();
+```
+
+See the full [API](https://github.com/xtermjs/xterm.js/blob/master/addons/xterm-addon-fit/typings/xterm-addon-fit.d.ts) for more advanced usage.

addons/xterm-addon-search/README.md

@@ -0,0 +1,23 @@
+## xterm-addon-search
+
+An addon for [xterm.js](https://github.com/xtermjs/xterm.js) that enables searching the buffer. This addon requires xterm.js v4+.
+
+### Install
+
+```bash
+npm install --save xterm-addon-search
+```
+
+### Usage
+
+```ts
+import { Terminal } from 'xterm';
+import { SearchAddon } from 'xterm-addon-search';
+
+const terminal = new Terminal();
+const searchAddon = new SearchAddon();
+terminal.loadAddon(searchAddon);
+searchAddon.findNext('foo');
+```
+
+See the full [API](https://github.com/xtermjs/xterm.js/blob/master/addons/xterm-addon-search/typings/xterm-addon-search.d.ts) for more advanced usage.

addons/xterm-addon-web-links/README.md

@@ -1,8 +1,6 @@
 ## xterm-addon-web-links
 
-[![Build Status](https://dev.azure.com/xtermjs/xterm-addon-web-links/_apis/build/status/xtermjs.xterm-addon-web-links?branchName=master)](https://dev.azure.com/xtermjs/xterm-addon-web-links/_build/latest?definitionId=5&branchName=master)
-
-An addon for [xterm.js](https://github.com/xtermjs/xterm.js) that enabled web links. This addon requires xterm.js 3.14+.
+An addon for [xterm.js](https://github.com/xtermjs/xterm.js) that enables web links. This addon requires xterm.js v4+.
 
 ### Install
 
@@ -20,4 +18,4 @@ const terminal = new Terminal();
 terminal.loadAddon(new WebLinksAddon());
 ```
 
-You can also specify a custom handler and options, see the [API](https://github.com/xtermjs/xterm-addon-web-links/blob/master/typings/web-links.d.ts) for more details.
+See the full [API](https://github.com/xtermjs/xterm.js/blob/master/addons/xterm-addon-web-links/typings/xterm-addon-web-links.d.ts) for more advanced usage.

addons/xterm-addon-webgl/README.md

@@ -0,0 +1,23 @@
+## xterm-addon-webgl
+
+An addon for [xterm.js](https://github.com/xtermjs/xterm.js) that enables a WebGL-based renderer. This addon requires xterm.js v4+.
+
+⚠️ This is an experimental addon that is [missing some features and may be unstable](https://github.com/xtermjs/xterm.js/issues?q=is%3Aopen+is%3Aissue+label%3Aarea%2Faddon%2Fwebgl) ⚠️
+
+### Install
+
+```bash
+npm install --save xterm-addon-webgl
+```
+
+### Usage
+
+```ts
+import { Terminal } from 'xterm';
+import { WebglAddon } from 'xterm-addon-webgl';
+
+const terminal = new Terminal();
+terminal.loadAddon(new WebglAddon());
+```
+
+See the full [API](https://github.com/xtermjs/xterm.js/blob/master/addons/xterm-addon-webgl/typings/xterm-addon-webgl.d.ts) for more advanced usage.