Replace README.txt with README.md

As per md format updated the read me file

Charchil Khandelwal → made their first commit to this issue’s fork.

Patch is working and marking it to RTBC.

It would be better if somebody else would review the MR.

Reviewed MR!1, the patch is not cleanly applied.

1. Use - (hyphen) in place of * (asterisk) in Maintainers section.
2. There is no need to specify the term "Current Maintainers " 

It is still not as per recommend format.
Please check

Headings capitalized with an initial capital, following standard English sentence rules

Bulleted lists denoted by dashes (-)

Reviewed MR!1, file as updated as per the readme.md file template and all the issues mentioned by @nupur, @rushiraval and @ apaderno are fixed, hence marking it to RTBC.

Please review. Thanks.

Reviewed MR!1, the following changes has been made.

1. For the given line "By clicking on this
tooltip, users will see the attached nodes in a Ctools modal windows." , plural words are used now.
2. Instead of "-> " using " > " now.

Changes are required here:
1." For that, use the the usual "Manage fields", In this line of code, " the" is used twice.