[DOC] add note about SDM

[jdkent]

references #183 and #591

Changes proposed in this pull request:

• add a note about the lack of implementation of SDM in the documentation

Relevant documentation: https://nimare--764.org.readthedocs.build/en/764/api.html

[codecov]

Codecov Report

Base: 88.64% // Head: 88.64% // No change to project coverage 👍

Coverage data is based on head (42b98c3) compared to base (282f197).
Patch has no changes to coverable lines.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #764   +/-   ##
=======================================
  Coverage   88.64%   88.64%           
=======================================
  Files          38       38           
  Lines        4324     4324           
=======================================
  Hits         3833     3833           
  Misses        491      491           

Impacted Files	Coverage Δ
nimare/io.py	95.14% <0.00%> (ø)
nimare/meta/utils.py	95.73% <0.00%> (ø)
nimare/transforms.py	75.36% <0.00%> (ø)
nimare/meta/kernel.py	95.78% <0.00%> (ø)
nimare/meta/cbma/ale.py	96.61% <0.00%> (ø)
nimare/meta/cbma/mkda.py	98.75% <0.00%> (ø)
nimare/decode/continuous.py	94.79% <0.00%> (ø)

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