Update intro and remove 'Application Period' column

[aaditkamat]

I've added removed the 'Application Period' column from the table as suggested by @luyangkenneth in #10 and also added a note with most of the stuff he has mentioned except for this point: "more kinds of internship roles may be offered than just the ones listed" since we have explicitly stated that the positions listed are for Software Engineering unless so mentioned in the Remarks column. Resolves #10

[sushinoya]

I'm still in favour of keeping the column. I've updated the issue for this PR. We can definitely phrase the description better though

[aaditkamat]

I'll add back the column. Are the other changes ok though?