Fine to put as an appendix to my statement of purpose the paragraph explaining the gaps in my transcripts? I am applying to doctoral programs in the US, and, because I have two breaks in my studies as what shown in my transcripts, I want to find somewhere to explain these. 

But I am concerned with that if I simply put this short paragraph explaining my breaks in studies somewhere in my application package as a supplemental material, then it may be ignored (would it?)! Thus I decided to include it in my statement of purpose as an appendix. 

The number of the words in my statement of purpose is not a problem, for even when the paragraph is included, my statement of purpose still contains less than 800 words.

So would such deed of mine have any chances to annoy any member of the committees?
This would be fine. One fundamental point of the personal statement is to allow you to comment on anything unusual in your records (gaps in education, a semester when you got sick and had low grades, etc.)   

In general, you can format the statement of purpose however you like. Using clear signposts (such as section headings) can make it much easier to follow. 

Remember that the person reading your personal statement probably has a large stack of them to read, so making their job easier can only help you.