Mercurial plugin: revise README

Closes #4616
Changes the `plugins=(... mercurial ...)` to be closer to correct zsh syntax, removing commas and extra spaces.
Revises discussion on configuring themes to avoid sounding like users should modify the theme definition file in the main OMZ folder. Prioritizes the suggestion of using a theme which already supports hg_prompt_info.

plugins/mercurial/README.md

@@ -2,23 +2,20 @@
 ### Usage
 Update .zshrc:
 
-1. Add name to the list of plugins, e.g. `plugins = (..., mercurial, ...)`
+1. Add name to the list of plugins, e.g. `plugins=(... mercurial ...)`
    (that is pretty obvious).
-2. Change PROMPT variable of current theme to contain current folder mercurial repo info:
+2. Switch to a theme which uses `hg_prompt_info`.
 
-   robbyrussel theme is used by default, so you need to modify PROMPT var
-   from [this file](https://github.com/robbyrussell/oh-my-zsh/blob/master/themes/robbyrussell.zsh-theme)
-   by adding `$(hg_prompt_info)` after `$(git_prompt_info)`, so currently it
-   looks next:
+   Or, customize the `$PROMPT` variable of your current theme to contain current folder mercurial repo info. This can be done by putting a custom version of the theme in `$ZSH_CUSTOM` or by changing `$PROMPT` in `.zshrc` after loading the theme.
 
-   ```diff
-   - PROMPT='${ret_status}%{$fg_bold[green]%}%p %{$fg[cyan]%}%c %{$fg_bold[blue]%}$(git_prompt_info)%{$fg_bold[blue]%} % %{$reset_color%}'
-   + PROMPT='${ret_status}%{$fg_bold[green]%}%p %{$fg[cyan]%}%c %{$fg_bold[blue]%}$(git_prompt_info)$(hg_prompt_info)%{$fg_bold[blue]%} % %{$reset_color%}'
+   The `robbyrussell` theme is used by default, so you need to modify `$PROMPT` var by adding `$(hg_prompt_info)` after `$(git_prompt_info)`, so it looks like this:
+
+   ```zsh
+   PROMPT='${ret_status}%{$fg_bold[green]%}%p %{$fg[cyan]%}%c %{$fg_bold[blue]%}$(git_prompt_info)$(hg_prompt_info)%{$fg_bold[blue]%} % %{$reset_color%}'
    ```
-   
-   and put modified var at the end of **.zshrc**.
+
 3. Initialize additional vars used in plugin. So in short put next in **.zshrc**:
-   
+
    ```
    ZSH_THEME_HG_PROMPT_PREFIX="%{$fg_bold[magenta]%}hg:(%{$fg[red]%}"
    ZSH_THEME_HG_PROMPT_SUFFIX="%{$reset_color%}"
@@ -55,8 +52,7 @@ Update .zshrc:
 #### Displays repo branch and directory status in prompt
 This is the same as git plugin does.
 
-**Note**: additional changes to **.zshrc** are required in order for this to
-work.
+**Note**: Additional changes to **.zshrc**, or using a theme designed to use `hg_prompt_info`, are required in order for this to work.
 
 ### Mantainers
 [ptrv](https://github.com/ptrv) - original creator