Add a v1 => v2 migration guide

docs/v2/manual.md

@@ -1424,6 +1424,166 @@ In this example the flag could be used like this :
 
 Side note: quotes may be necessary around the date depending on your layout (if you have spaces for instance)
 
+### Converting from v1 to v2
+
+v2 has a number of breaking changes but converting is straightforward:
+make the changes documented below then resolve any compiler errors.
+Typically this is sufficient.
+
+If you find any issues not covered by this document, please post a
+comment on [Issue 921](https://github.com/urfave/cli/issues/921) or
+consider sending a PR.
+
+#### Flags before args
+
+In v2 flags must come before args.  This is more POSIX-compliant.  You
+may need to update scripts, user documentation, etc.
+
+This will work:
+
+```
+cli hello --shout rick
+```
+
+This will not:
+
+```
+cli hello rick --shout
+```
+
+#### Import string changed
+
+* OLD: `import "github.com/urfave/cli"`
+* NEW: `import "github.com/urfave/cli/v2"`
+
+Check each file for this and make the change.
+
+Shell command to find them all: `fgrep -rl github.com/urfave/cli *`
+
+#### Flag aliases are done differently.
+
+Change `Name: "foo, f` to "Name: "foo", Aliases: []string{"f"}`
+
+OLD:
+
+```
+cli.StringFlag{
+  Name: "config, cfg"
+}
+```
+
+NEW:
+
+```
+cli.StringFlag{
+    Name: "config",
+    Aliases: []string{"cfg"},
+}
+```
+
+Sadly v2 doesn't warn you if a comma is in the name.
+
+#### Commands are now lists of pointers
+
+Occurrences of `[]Command` have been changed to `[]*Command`.
+
+What this means to you:
+
+Look for `[]cli.Command{}` and change it to `[]*cli.Command{}`
+
+Example:
+
+* OLD: `var commands = []cli.Command{}`
+* NEW: `var commands = []*cli.Command{}`
+
+Compiler messages you might see:
+
+`
+commands/commands.go:56:30: cannot convert commands (type []cli.Command) to type cli.CommandsByName
+commands/commands.go:57:15: cannot use commands (type []cli.Command) as type []*cli.Command in assignment
+`
+
+#### Lists of commands should be pointers
+
+If you are building up a list of commands, the individual items should
+now be pointers.
+
+* OLD: `cli.Command{`
+* NEW: `&cli.Command{`
+
+Compiler messages you might see:
+
+`
+./commands.go:32:34: cannot use cli.Command literal (type cli.Command) as type *cli.Command in argument to
+`
+
+#### cli.Flag changed
+
+`cli.Flag` is now a list of pointers.
+
+What this means to you:
+
+If you make a list of flags, add a `&` in front of each
+item.   cli.BoolFlag, cli.StringFlag, etc.
+
+* OLD:
+`
+        app.Flags = []cli.Flag{
+               cli.BoolFlag{
+`
+
+* NEW:
+```
+        app.Flags = []cli.Flag{
+               &cli.BoolFlag{
+```
+
+Compiler messages you might see:
+
+```
+	cli.StringFlag does not implement cli.Flag (Apply method has pointer receiver)
+```
+
+#### Appending Commands
+
+Appending to a list of commands needs to be changed since the list is
+now pointers.
+
+* OLD: `commands = append(commands, *c)`
+* NEW: `commands = append(commands, c)`
+
+Compiler messages you might see:
+
+`
+commands/commands.go:28:19: cannot use c (type *cli.Command) as type cli.Command in append
+`
+
+#### Actions returns errors
+
+A command's `Action:` now returns an `error`.
+
+* OLD: `Action: func(c *cli.Context) {`
+* NEW: `Action: func(c *cli.Context) error {`
+
+Compiler messages you might see:
+
+```
+./commands.go:35:2: cannot use func literal (type func(*cli.Context)) as type cli.ActionFunc in field value
+```
+
+#### Everything else
+
+Compile the code and worth through any errors it finds. Most should
+relate to issues listed above.
+
+Once it compiles, test the command. Make sure `-h` displays help
+properly, then try various flags and subcommands.
+
+If you find anything not covered by this document please let us know
+by submitting a comment on
+[Issue 921](https://github.com/urfave/cli/issues/921)
+so that others can benefit.
+
 ### Full API Example
 
 **Notice**: This is a contrived (functioning) example meant strictly for API