docs(sed): document sed syntax, quoting, and escaping slash (#170)

No change to logic, only docs.

This documents the special sed syntax (ex. `shx sed s/foo/bar/g
filename.txt`). With that, this mentions that `/` is a special
character, so the user must escape that if it's present in either the
regex or replacement string.

This also documents how Windows handles single quotes differently, so
it's desirable to avoid single quotes in package.json scripts. This part
isn't specific to sed, but since it's popular to quote sed expressions,
I'm adding this to the docs at the same time.

Fixes #165
Fixes #169

README.md

@@ -80,9 +80,44 @@ However, `shx` is ideal for one-liners inside `package.json`:
 }
 ```
 
-## Unsupported Commands
+**Tip:** because Windows treats single quotes (ex. `'some string'`) differently
+than double quotes, [we
+recommend](https://github.com/shelljs/shx/issues/165#issuecomment-563127983)
+wrapping your arguments in double quotes for cross platform compatibility (ex.
+`"some string"`).
 
-Due to the differences in execution environments between ShellJS and `shx` (JS vs CLI) some commands are not supported:
+## Command reference
+
+Shx exposes [most ShellJS
+commands](https://github.com/shelljs/shelljs#command-reference). If a command is
+not listed here, assume it's supported!
+
+### sed
+
+Shx provides unix-like syntax on top of `shell.sed()`. So ShellJS code like:
+
+```js
+shell.sed('-i', /original string/g, 'replacement', 'filename.txt');
+```
+
+would turn into the following Shx command:
+
+```sh
+shx sed -i "s/original string/replacement/g" filename.txt
+```
+
+**Note:** like unix `sed`, `shx sed` treats `/` as a special character, and
+[this must be
+escaped](https://github.com/shelljs/shx/issues/169#issuecomment-563013849) (as
+`\/` in the shell, or `\\/` in `package.json`) if you intend to use this
+character in either the regex or replacement string. Do **not** escape `/`
+characters in the file path.
+
+### Unsupported Commands
+
+As mentioned above, most ShellJS commands are supported in ShellJS. Due to the
+differences in execution environments between ShellJS and `shx` (JS vs CLI) the
+following commands are not supported:
 
 | Unsupported command | Recommend workaround |
 | ------------------- | -------------------- |