Document that 'cargo install' is needed to reindex

Jenkins job DGRPH/gitlab-builds #2669 failed in 7 min 42 sec.
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wtf, there is no failure in the logs

@jenkins retry build

Jenkins job DGRPH/gitlab-builds #2670 succeeded in 5 min 36 sec.
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added 4 commits

• f5a569a5...42b4d435 - 3 commits from branch master
• beb0fb81 - Document that 'cargo install' is needed to reindex

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merged

Jenkins job DGRPH/gitlab-builds #2692 succeeded in 6 min 58 sec.
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