Note why we don't implement TFCE in NiMARE (currently)

[tsalo]

Closes #145 and closes #655.

NOTE: I still hope to implement a fast probabilistic TFCE method in the future.

Changes proposed in this pull request:

• Add a note to the documentation about why we're not going to implement TFCE in NiMARE.

[codecov]

Codecov Report

Merging #680 (0de68a7) into main (b186fb4) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             main     #680   +/-   ##
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  Coverage   85.31%   85.31%           
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  Files          41       41           
  Lines        4528     4528           
=======================================
  Hits         3863     3863           
  Misses        665      665           

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