Document need of deepmerge for future reference

packages/hurumap-core/README.md

@@ -0,0 +1,15 @@
+# @hurumap/core
+
+### Getting started
+
+To run `@hurumap/core` test:
+
+```sh
+pnpm test
+```
+
+### Development
+
+#### Important
+
+While `@mui/utils` provides a [`deepmerge`](https://github.com/mui/material-ui/blob/next/packages/mui-utils/src/deepmerge/deepmerge.ts) function, this function doesn't support merging arrays. For this reason, `@hurumap/core` opts to use [`deepmerge`](https://github.com/TehShrike/deepmerge) package since HURUmap makes have use of arrays of data to render charts (using Vega library).

packages/hurumap-next/README.md

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+# @hurumap/next
+
+### Getting started
+
+To run `@hurumap/next` test:
+
+```sh
+pnpm test
+```