docs: Add documentation explaining how to enable Seshat support.

docs/native-node-modules.md

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+Native Node Modules
+
+Since v???, the electron version of Riot can make use of native node modules.
+These allow Riot to integrate with the desktop in ways that a browser cannot.
+
+While handy, these modules must be compiled and are thus downloaded
+pre-compiled during build so that a single OS can compile Riot for all
+platforms. If you would like to compile the native node modules from source,
+as is done for Riot releases, instead of trusting binaries hosted on npm,
+then please read on.
+
+Do note that compiling a module for a particular operating system
+(Linux/Mac/Windows) and will need to be done on that operating system.
+
+## Adding Seshat support
+
+Seshat is a native node library that adds support for local event indexing and
+full text search in E2E encrypted rooms.
+
+Since Seshat is written in rust the rust compiler and cargo tool-chain need to be
+installed before installing Seshat itself. After installing the compiler Seshat
+support can be added using yarn inside the `electron_app/` directory:
+
+    yarn add matrix-seshat
+
+After this is done the electron version of riot can be run from the main folder
+as usual using:
+
+    yarn electron
+
+If for some reason recompilation of Seshat is needed, e.g. when using a
+development version of Seshat using `yarn link`, or if the initial compilation was
+done for the wrong electron version, Seshat can be recompiled with the
+`electron-build-env` tool. Again from the `electron_app/` directory:
+
+    yarn add electron-build-env
+
+Recompiling Seshat itself can be done like so:
+
+    yarn run electron-build-env -- --electron 6.1.1 -- neon build matrix-seshat --release`
+
+Please make sure to include all the `--` as well as the `--release` command line
+switch at the end. Modify your electron version accordingly depending on the
+version that is installed on your system.