Add pages to discuss flow dev guide

[LEEHYUKJOO]

In my company, my colleagues use Node-RED broadly. And we have written flow development guidelines for developers.
We think these contents are useful also for all Node-RED users. So, I think that it would be better to prepare new pages or sections on Node-RED web site for providing these practices.

I edited the flow development guideline to markdown file for discussing chapter composition.
This PR is for adding documents as official doc of Node-RED.

Ask

Please comment if you have some ideas or opinions.

1. Changing the order of the chapters
2. Removing unnecessary chapters
3. Chapters that does not matched to title
4. etc.

Contents

These files only contain chapter composition. The descriptions of each section are written as italic font.
I have not added specific contents of this flow development guideline yet.
The contents of each section will be added after finished the discussion about chapter order at end of February.
I think that it is more efficient to edit and summarize guideline that order.

I expect other users to share their opinions about these documents.

[knolleary]

Thanks @LEEHYUKJOO

I'm not sure Practice is quite the right title for this new section. I'd suggest Developing Flows would be more consistent with the other sections.

No need to change anything right away - as discussed, once we have more content in the guide I think it will be easier to identify the right way to integrate it with the existing docs. What you have here is a good start begin collaboration on it.

I'll merge this now and raise a follow-up issue regarding the naming which we can resolve at a later date before merging to master.

[natcl]

Perhaps "Best practices" would work?

[knolleary]

@natcl fyi #93 raised for the naming discussion