Update docs/index.txt
Update docs/index.txt

Clarify that strict 4 space indentation applies to nested lists. 

Closes #193. Hopefully, people will read this in the docs before
filing bug reports in the future. One can dream.
gogobook/Python-Markdown,zestedesavoir/Python-ZMarkdown,fernandezcuesta/Python-Markdown,zestedesavoir/Python-ZMarkdown,Situphen/Python-ZMarkdown,fernandezcuesta/Python-Markdown,me-and/Python-Markdown,me-and/Python-Markdown,evertqin/Python-Markdown,fernandezcuesta/Python-Markdown,cyisfor/Python-Markdown,me-and/Python-Markdown,gogobook/Python-Markdown,Situphen/Python-ZMarkdown,waylan/Python-Markdown,dataquestio/Python-Markdown,Situphen/Python-ZMarkdown,joachimneu/Python-Markdown,cyisfor/Python-Markdown,evertqin/Python-Markdown,cyisfor/Python-Markdown,evertqin/Python-Markdown,dataquestio/Python-Markdown,waylan/Python-Markdown,joachimneu/Python-Markdown,joachimneu/Python-Markdown,zestedesavoir/Python-ZMarkdown,dataquestio/Python-Markdown,gogobook/Python-Markdown