[doc] dev: add test DB change

doc/dev/test-db-change.md

+# How to test a change in the schema of a DB
+
+1. Test your code first via automated tests
+2. Compile duniter
+3. Compile dex
+4. Create a duniter profile for your test
+5. Synchronize a small blockchain for your test
+6. Explore the DB with dex to verify that it contains the expected data
+
+## 1. Test your code first via automated tests
+
+## 2. Compile duniter and dex
+
+There are several ways to compile duniter depending on what you have modified.
+
+If all your diffs are only in the `rust-libs` folder, then the fastest way is to run the neon script directly:
+
+`./neon/build.sh`
+
+WARNING: this only works if you have already compiled duniter completely since your last update on the `dev` branch.
+
+If you have diffs in other folders than `rust-libs`, or if in doubt, recompile everything:
+
+`cargo xtask build`
+
+## 3. Compile dex
+
+`cargo bdex`
+
+WARNING: this take a long time!
+
+## 4. Create a duniter profile for your test
+
+There is no profile creation command. The profile is automatically created the first time it is used.
+
+This step is only necessary if you need your test profile to be a different configuration than the default.
+
+For example, if you need GVA, use the following command:
+
+```bash
+$ cargo rr -p test1 wizard gva
+    Finished release [optimized] target(s) in 0.06s
+     Running `target/release/duniter -p test1 wizard gva`
+2021-03-14T22:34:47+01:00 - warn: No configuration loaded
+Enable GVA API? [Y/n]
+Listen to ip v4 ? [127.0.0.1]
+Listen to ip v6? [Y/n]n
+Listen to port ? [30901]
+Path ? [gva]
+Subscriptions path ? [gva-sub]
+Define a remote host? [y/N]
+Define a remote port? [y/N]
+Define a remote path? [y/N]
+Define a remote subscriptions path? [y/N]
+Update whitelist? [y/N]
+Configuration successfully updated.
+```
+
+## 5. Synchronize a small blockchain for your test
+
+You will need to have the json archives of a blockchain.
+The easiest way is to take the json archives from the beginning of Ğ1, you can find them on the gitlab in the `blockchain-archives` repository:
+
+[link to the json archive of the first 132446 blocks of the Ğ1](https://git.duniter.org/c-geek/blockchain-archives/-/blob/master/snapshots/g1/132446.tar.gz)
+
+Then, start a synchronization with the following command:
+
+`cargo rr -p test1 sync /path/to/json/archive`
+
+If the synchronization is too long, there are two solutions:
+
+1. Recompile duniter in release mode (`NEON_BUILD_RELEASE="true" ./neon/build.sh`)
+2. Reduce the size of the json archive by deleting chunks from the end
+
+## 6. Explore the DB with dex to verify that it contains the expected data
+
+See `./target/release/dex --help`